One heartbeat is generally two seconds. However, it's said to vary sometimes with lag.
There are a few things that happen on each heartbeat:
- You get a small amount of xp, known as "heartbeat xp". This starts at 3 xp per heartbeat and is decreased as your xp rate gets above 500,000 xp per hour.
- If injured, you will also--usually--regenerate a small number of hp. The exact number depends on your constitution and strength, varying from 2 to 6 within the range of possible unmodified stats, with 4 being the rate that all 13's gives you. With very damaged stats, you may have a regeneration rate of 0.
- If you have less than full gp, you will regenerate a small number of gp. The exact number depends on your guild and stats (the specific stats it depends on varies with guild). Natural regeneration rates vary from 1 to 4, with 3 being the rate that all 13's gives you. With very damaged stats, you may have a regeneration rate of 0.
- If you have less than full social points, you regenerate one.
Oddly enough, being dead doesn't seem to stop your heartbeat, as your xp and gp still increase while you are a ghost (however, this is only visible with certain clients or plugins that can update from mxp, since being dead means you cannot view your score).
Apparently, having a heartbeat is required in order to take part in fights.
Some effects, such as Major Shield seem to have a duration measured in heartbeats.
Restarting your heartbeat
The "restart" command restarts your heartbeat if it is stopped by a bug.