I put four weeks, and I use to not charge board, but have had horses here that sold and were still here (I was paid in full), that the people backed out of, and I ended up (depending on the contract) refunding or crediting or keeping the money, but now have a horse that needs to be remarketed for sale.
If people are timely and responsible, I don't get to hard nosed, but I've had some flakes that don't pay or leave horses for months, then change their minds. I don't charge alot for board, but our time does have some value, as well as the feed, etc.
Weanlings - they're board free until weaned. Then I would give a a few weeks to a month before charging board. Same for other special circumstance horses - Christmas presents bought in the Fall, I don't generally charge board, and depending on the price I don't charge for farrier/deworming either. Our farrier is $25 a head.