You need a minimum of 4 weeks to get mature seeds. Five to six weeks is best (I always do 6 just to make sure.) You can always harvest the other buds on the plant if it is just a single branch that was seeded. I usually leave mine on for another week or two after the rest of the plant has been harvested to ensure they are all matured. If you care primarily about the seeds, you shouldn't even be bothering with what the trichs look like. Good chance they will be amber or even brown by the time the seeds finish. If you are not sure if they are done or not you can always remove a single seed and see if it is mature or not.

If your main concern is the bud, then harvest when the trichs look like they should when you always harvest. If there is any chance that the seeds came from a hermi, don't bother to save them. Any seeds that comes from a hermi can carry the hermi trait and should not be propagated.