The temperature determines the length of time the GP relay provides current to the glow plugs. On my 240D when cold the relay stays on about 20 seconds.

If the glow plugs are .6 ohms and the voltage is 12 volts that equals 20 amps per plug. I think in reality it ends up being a little less as the fuse in the relay is rated for 80 amps. If all four plugs drew 20 amps the fuse would blow.

JV