If i'n speedrunning a lengthy game and i need to take a break for 5 minutes, can I pause the run and resume the timer when i get back? or do i have to do it in one session?

Every game has its own rules for this sort of thing, you'll have to check with the specific game you're planning on running.

Generally, pausing the timer is frowned upon for games timed with real-time unless the game's rules say otherwise.

Depending on the game too you might be able to find a "break point" anyways. I've got one in Lost Legacy doing permadeath attempts if I get up to saving the elephant, that's a 2-3 minute autoscroller I can use for a bathroom break.

If the category is RTA, or Real Time Attack, the timer cannot be paused. If the category is Time Attack, it may have its own specific rules on pausing and breaks. The vast majority of speedrun categories are treated as RTA, though.