The minion mine actually maxes out at 8 mins. I recommend only taking one unless you team is pushing turrets. I'm that situation, taking both mines would be more advantageous. In the late game, taking minion mines is a win-win situation. If your team is winning, taking both and pushing makes it harder for the enemy team to make a comeback. If your team is losing, taking a minion mine will cause the enemy minions to deal less damage. Watch out for enemy heroes though, they probably will camp...