Our initial idea was that each donation pays for 30 days of tips (and rewards depended on interval between commits, see http://blog.anonymousads.com/2013/10/tip4commit-first-bitcoi...). But that idea appeared to be more complicated and a bit unfair (consecutive commits could receive very different amounts). So we switched to a simpler one. Yes, mathematically speaking, there will always be some money in our wallet. But we don't expect to have huge amounts of money there since donations are never stable and some projects have dozens commits daily.