$100/month/each for "guilt-free" spending on about $5700 take home. Basically we spend it on things that only benefit us individually and not the family as a whole, such as me going to the bar with the guys, buying lunch at work instead of packing, dues / expenses related to social organizations she belongs to, etc. We also use it to buy things that we can't agree on as far as it being a 'necessity'. If one of us can't convince the other of a sizable purchase or know it is truly a luxury item, we have the option of saving up and spending our allowance on it. For example, my wife wanted some really expensive boots last fall and couldn't decide between two pairs, so she kept both and paid for them out of her allowance instead of taking the hit against our clothing budget. I tend to spend about all of it every month, she is a saver and makes a handful of larger purchases through the year.
We didn't have the allowance in place prior to her becoming a SAHM. Before she quit her job, we had plenty of excess income and spent responsibly in general so occasional splurges were not an issue. She had a little difficulty adjusting to a more strictly budgeted, single-income mindset, and actually requested that we set an allowance for each other to help her out. I've proposed eliminating them now that we've been successful for the last couple of years, and the kids act as a natural throttle on the amount of opportunities we have to spend money on ourselves, but she still likes having a concrete number to plan with.