If it can be implemented relatively reasonably, a progressive delay system would seem fairest, because it recognizes that some people realize there's a mismatch between them and guild in terms of how much they participate etc., and/or they get kicked.
TL;DR version:
First/second guild hop should be allowed as short as they are now, but after that it should be progressive - for instance after the next guild hop there's a 2-day cooldown, after the next a 3-day, etc. (or whatever other interval seems fair) but with the extended cooldown dropping back when players don't guild hop.
End TL;DR - more details/alternatives below.
Alternatively, a maximum 5 fights per day regardless of multiple raids or guild hops for the first day, unless it's multiple raids (or heroic raids) in a non-guild-hopping situation (i.e. someone who has guild-hopped and a new raid starts, if they've already had 5 fights in a prior guild, they can't fight until the next raid; if they had 3 fights that day in a prior guild, they'll only have 2 left until next day) could work.
Second, if you're kicked out of or quit a guild before the raid is done, and collect no rewards, then there shouldn't be the same lengthy cooldown required (but in this case perhaps the maximum 5 fights per day for guild-hoppers should apply for the first day, to prevent someone with very strong teams jumping from guild to guild, doing high damage then quitting in a cycle to benefit a small groups? If that behaviour is a concern).
