I don't like the statement from aperson1. Just because a server load fluctuates doesn't mean that the pace of the game also fluctuates.
A couple of examples:
In EVE they have time dilation, so if tons of people are in a fight, the pace just slows to a crawl and everyone gets turns to do their actions or whatever. It's actually pretty interesting (considering that people outside that fight are moving in normal time, considering the differing ability of teams to use that extra time productively by giving direction, etc.)
In warcraft their "ticks" basically means like every half a second or whatever, damage is dealt. It used to be slower than it is but they sped up the ticks a few years ago.
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I think it's fair to say in LoL that a tick is at maximum half a second, because the health and mana regen give out their stats at that rate. I can't tell you any more than that, though.