Frequently.  When I am close to my proshop, I use Ebonite Powerhouse ball cleaner on my Ball spinner.  When I am away, I use the Ebonite Ball wipes.

I would recommend that you wipe off your ball after every shot and then clean it after every league session.  You spent the money on your equipment, why not protect your investment and allow it to work as best it can for as long as it can.

After bowling - every 3 games for leagues - could be as many as 12 games in practice.  I also wipe the ball down between shots with a microfiber towel - try to do that after every shot.  I tend to track close to fingers and thumb, so sometimes the oil gets too close to my index finger.

I use simple green/water/rubbing alcohol mixture, Ebonite Energizer cleaner, Brunswick Bowling ball cleaner (from Kmart), Storm Reacta Clean, Ebonite 2 part reactive ball cleaner (from Dick's sporting goods), and sometimes an Orange based degreaser - depends on what I have handy at the time.  Usually it's the simple green mixture in my bag.
--------------------
FAQ:http://www.ballreviews.com/Forum/Replies.asp?TopicID=74110&ForumID=16&CategoryID=5

Search Ballreviews entire database here: http://www.bowling-info.com/Search.html

Everytime I use my bowling ball, whether it is one shot or multiple games. I try to clean the ball at the lanes with Storm Recta clean or Ebonite Powerhouse. Then, after I get home I reclean and deep clean with clean n' dull. May be trying Legends cleaner soon. Sometimes I just use clean n'dull at the center but it's not as easy as most cleaners to use. More you take care of your bowling equipment, the longer it will last.

Chuck

After every session I clean my reactive balls that have been thrown one or more times.  I don't clean my spare ball hardly ever.

It's good to make a habit of cleaning them.  They really don't lose any or much  of their magic if you take care of them well.  Also, cleaning them when you get home or after your sessions at the alley gives you a good time to think about your bowling while it's fresh in your head.  I find being able to step back a little and thinking about your game really pays off.  Ball cleaning time is as good as any.

I use basic cleaners.  Glorified Windex essentially.  I have some clean and dull but I don't use it too much.

After each session.