We generally try to write a week or two before we got there, but we're very honest about having no idea when we will show up. We had some write back and say, "We'll be out of town X,Y, and Z days, but if you come other times you're welcome to stay." That helps us plan as we know we either need to get there before or after that. So far, all the hosts we've stayed with have been very flexible and have understood that too many things can happen on a bike to be able to plan way far in advance.

There was only one time when we changed plans at the last moment and ended up in a town we hadn't planned on - wrote to some warmshowers hosts there the day before, but didn't hear back. When we write in advance, we almost always hear back.