this depends on the host entirely. each one is different. the hosts i have experienced have expressed to me they prefer ~4 week stay agreements, because many shorter stays can feel more draining to them (so much time is spent on familiarizing yourself with the place to be able to then work effectively and know what they're doing), but it's also not so long that they feel hesitant to agree to (many hosts may feel uncertain to agree to let a stranger live with them for an entire growing season without first knowing if your personalities mesh well). but still, you'll see hosts that seem to prefer to host short stays only, and you'll also see hosts that are only looking for people to stay for several months.

as for spare money, bring enough to last you should you experience an emergency and have to leave a host's place unexpectedly. i have had experiences where i didn't really have to spend a penny for my entire stay, except for personal recreational stuff. i still wouldn't want to be caught without the funds to get myself out of a situation, find another to stay, feed myself, and go home in case something went wrong. (this is less a concern with WWOOF than it is with workaway in my experience, because of workaway's relatively shitty review system, but still. common sense to be prepared when traveling and doing this kind of thing.)