There's no good rule for when it is or is not appropriate to surrender.
Last night, I played a losing game. Every lane got beat to an extent, and they got several towers on us. It wasn't quite 6-26 at 20 minutes, but it was probably about 12-26 at that point.
The mid/top duo attempted to surrender, and when the other three of us voted no, they AFKed. But the enemy was sloppy and their team comp was bad (no tanks), so were actually able to 3v5 ace them on our Nexus to halt the bleeding. We were poised to continue winning team fights and stage a comeback. Too bad our mid and top were AFK by that point. With zero Nexus towers remaining, the enemy team simply suicided in to finish off our Nexus to win. With a full team of 5, I think we had a decent chance of holding out.
League is a game where if you score an ace after the ~30+ minute mark, you're going to gain some serious objectives. Hell, at a certain point, an ace means you simply win the game, even if they still have half their towers up. So unless the enemy is objectively impossible to answer, a surrender is kinda just laziness.