Last year, juggling uni commitments, I was lucky to be able to do 18 hours... an hour and a half a month. Barely able to stay current, and I think that'd have to be close to the bare minimum needed... This year I've got a little more disposable income so I can afford an hour every week and a half on average. Much more better.
It's an interesting question actually. Recency has been shown, many times, to be a key factor in accident risk or otherwise. As a private pilot, self-funding all my flying, it's hard to strike the right balance between not breaking the bank but flying enough to feel competent.