How do I disable time limit in TF2?

If all you want to do is screw around on the map, try the following command in console.

mp_tournament 1; mp_tournament_restart

For added fun and games, you can bind it to a key like so: bind <key> "mp_tournament 1; mp_tournament_restart", where <key> is the scripting name for the desired key.

This will enter tournament mode, which will leave the game suspended waiting for a tournament round to begin (and you can just never start that).

The mp_timelimit command operates on the map timer, which determines how long you have before the server changes maps. You are looking for a way to change the round time, not the map time.

For CTF maps, the round time equals the map time. For other maps the round time is strictly based on how well the two teams are doing: capping a point gives them more time. I know of no way to alter the round timer, as this would easily allow server owners to extend Attack/Defend maps forever to guarantee a BLU win, or stop stalemates from resolving on some PLR maps. If you want to fool around on a map alone, you'll need to keep capping points once time's running out to give yourself more time - and when the map's not symmetrical, one side will win eventually no matter what you do.

You can of course decompile the map and edit the timer, but that's probably not what you want.