...for another rule of thumb, in video post-processing 101 here they teach 1 second for each 5 characters, also not including spaces and punctuation.
If fading in/out title, fade period counts for half its duration (i.e. your text is supposed to disappear at 50% transparency)
ADDING that since credits are not supposed to be really read, you may select to go much faster and write much smaller since only fans will pick up what they like, for the rest it's only to please the people and associations involved. But legal warnings (usually at start) should follow the rule.
Out of topic: Definitely wise for tiltles anyway, the big red print being:
everything must be readable at once on the screen, and before the end of the clip. No scenes overlap, no half sentence, finishing on the next picture etc.
Caveat: art and avant-garde movies don't follow ANY rule.