While I find it legitimate to pause an online multiplayer match because of an emergency, an important phone call, the door bell or even for a bathroom break in a very long match, I feel that there are some players who don’t respect their opponent’s time and pause for longer than necessary or for illegitimate reasons.
I don’t want to start a discussion on when it is legitimate to pause and for how long. I can only say that I personally respect my opponent’s time and treat them the way I would like to be treated, so in over 2000 matches I probably haven’t paused for more than 10 minutes in total.
I have also never paused the match just to take a look at the tech tree, because I consider it to be a part of skill to know the civ bonuses, and if I need to look it up I do it during the running match because it’s not my opponent’s fault.
I am not going to suggest a limit for pausing or anything. My suggestion would be that you track a player’s pause time in online matches and whenever they pause, the other players can see their pause statistics (e.g. how many minutes of pausing per match, on average). That way players are discouraged from abusing it, and other players can make an educated guess if their opponent genuinely needs a moment, or if they are just a notorious pauser.