Hmm, I do regular budgeting between every source of income I get (I do freelance so I don't have monthly paychecks) - I don't have a fixed amount of spare but what I do is the 50-20-30 rule!
Whatever your income is, and for whatever time period, 50% would go to needs like bills and rent payment, 20% to savings and 30% to wants (aka spend however you wish on shopping, indulging etc).