This topic has come up in several of my readings today. While I’ve seen several strategies for what to do when your planning game is going too long, the best one is perhaps the simplest, and comes from none other than Ron Jeffries on the Scrum Development mailing list:
I suspect you’re going into way too much detail. And if you
over-commit, so what? Learn from it.
Try planning poker.
Or, maybe try an experiment. For each story, as soon as it is read,
everyone writes their estimate on a card and puts it face down.
After a few minutes of discussion, write down another estimate and
put it face down. (Mark the cards so you’ll know which is first
etc.) At the end of “enough” discussion, make a third card.
Take the average of the first cards, of the second cards, and of the
third cards. If the averages do not vary widely, what does that
suggest? If they do, note how they vary. Figure out why.