[relknes]

A frequent problem after opening 1♥ or 1♠ with a 6 card suit and hearing some response from partner is "Should I show my 4 card minor, or rebid the major to show 6 of them?"
Personally, I show the second suit the majority of the time on the theory that it is better to let partner know about 9 cards (5+ in the major and 4+ in the minor) than letting them know about 6 cards (6+ in the major). I only rebid the 6 card suit if the suit is solid or semi-solid (say, AQJxxx) and the minor is very weak (say, Jxxx or worse). Partner will usually correct back to the major with a doubleton unless they have 4 card support for the minor. If I have 6 spades and 4 hearts, I always show the hearts with my second bid, regardless of the strength difference.
What are other people's criteria?

[billw55]

I check my valuation of the hand as a whole, in context of partner's response. If I rate it a minimum opener, I usually rebid the six bagger, to limit my hand quickly. If I think it is somewhat better, worth a third bid, I will go 6-4-6. It can also matter if partner's first response is NF, F1, or FG.

[aguahombre]

Have lived to the tender age of 68 without ever using a conundrum. Probably should have with my ex.

[the_clown]

I tend to show my second suit 100% of the time if I have extra values.

With minimum hands it depends on the form of scoring and suit quality.

[Zelandakh]

Europeans almost always show the second suit in my experience. Americans often seem to be taught to rebid the major with a minimum, although my perception from over here is that there is a distinct move towards rebidding the minor there too (unless the minor is very weak and/or the major very good).