@sus49 - LOL - I know!   There can be no hard-and-fast rule - how much gets lost from the bow will depend on a lot of factors and pieces you play.  Bow speed and pressure will most likely affect just how much gets thrown off during playing.  It will, also depend on the rosin itself - some are by design/source much more sticky than others.  It will also depend on the bow hair, its general "quality" and indeed its age (in the sense of how long, and how hard it has been used).   I "sort of suspect" that the strings also have an effect, be it the winding or material, although I can't say for sure.  

But all these things add to the "when do I need to apply more rosin" question.   There are hints in what both @DanielB said above, and the text that goes with @Fiddlerman 's video.

I begin to suspect that your question may be better framed as "When should I..." rather than "How often..."

Read on - try not to fall asleep ( kidding! ) - 

I did an experiment in my early days of playing ( listen to me, "early days" LOL - I'm only just over a year in ) but I did this with a spare bow I had - EVERY time I played I rosined the bow, and would "play" ( well, at that stage, make noises ) for around 15 to 20 minutes.  And each time I applied rosin, I gave it five full bow strokes.  NEEDLESS TO SAY, after around, oh, maybe 10 or twenty times of doing this I was gradually becoming aware of a change, not only in bowing, but of tonal quality (bad though it was anyway with my bowing) up to the point where yes, I actually saw a cloud of dust disengage itself from the bow while bowing fortissimo ! LOLOL ( hence my "misty-haze" thoughts for the poll question ).  THAT, I knew was WAY TOO MUCH ( and remember, I did this on purpose, as an experiment.  I had a separate bow which was much more sparingly rosined ).

Rosin ALSO builds up, on the strings - again, no definite way of saying "how or when" because of the different rosin types' stickiness, and the string material/winding.  If it is noticeable on the strings, I advise removing it, because it WILL affect the tonal quality.  Remove it with a cloth, or a wine-cork with a little slot cut into it.  You may choose to apply some denatured alcohol on the cloth, but don't soak it and don't get it on to the varnish of the body.

A possible answer to your question might be framed like this - Start off with a bow that you "feel" is sufficiently well rosined.  ( There may be way too much - no matter - in fact for this experiment it is probably a good idea to be over-loaded! ).   I don't know how long your individual practice sessions are - let's say 30 minutes.  Before you play, gently dust off the strings at the contact points between the bridge and fingerboard.  No need to use the alcohol dampened swab, just make sure there is no excessive amount caking up on the string.  Play your practice session without rosining the bow.   Next time you play, just check the strings again - do they look powdery?  Yes - OK - just gently wipe 'em down - and do NOT apply more rosin.  Play your session.  Repeat.  Repeat.  Repeat until you start becoming aware of the fact that the bow is no longer "biting" the strings, or, it is biting, but you feel you are quite specifically needing to apply more pressure - and without the additional pressure you "feel" the bow is starting to "glide" over the strings rather than "bite into them"

That would be past the point of the "When" to apply rosin again.     The "How often" cannot be easily answered - it really depends on so many things I referred to somewhere above, it is impossible to give a standard answer.  Very specifically, for me, with the bow I have, the rosin I use,  the strings I have fitted, and the type of playing I do (which varies a lot, from gentle legato "ppp" to hard-bowed and very active martele strokes, and heavy-ish pressure as I'm practising drones and double stops at the moment - I find ( roughly - my ears and the general feel tell me REALLY when ) that with an average of 1.5 hours per day playing, I will re-apply rosin probably around every 3 to 4 days - so the "how often" for me would be between 5 and 6 hours, with my "light" pressure during re-rosining.  When I re-apply rosin, the  bow strokes over the rosin cake are "relatively light" ( like, I am not actually pushing the hair all the way down to the stick ).

But, the truth about it is - I'm lazy during practice - and if I'm going to do a "performance" ( live on StreetJelly or a video to post here ) - I will ALWAYS stop, check the build-up (if any) on the strings, and if it has been more than a few "playing hours" since last applied, I'll apply a gentle re-rosining to the bow - it PROBABLY doesn't need it - but I know it will NOT overload it, and it WONT throw up clouds of dust.   

It is a very "personal" thing, I think - depends so much on the kit you have ( bow hair, rosin type, string type, playing action, type of music you play and the bowing-action such music demands )

Do you have a spare bow - if so you could repeat my experiment - it might be enlightening.

Sorry for the "treatise" - I'm just trying to describe in my own words "what I found myself" on the journey.... and hope the over-detailed explanation of what I uncovered on my journey didn't send you to sleep.....

BillyG