Cranking speed is very important.
An old, "tired" starter will eventually get to the point where it will not turn the engine fast enough to get it started, just like the effects of a weak battery. This will become most evident in cold weather, of course. Especially if your engine has a lot of miles and lower compression, or the glow plugs aren't exactly new. Cranking power is much more critical in cold weather...obviously the engine will be more difficult to start....the oil is colder and doesn't flow as well....batteries will typically not perform at their peak....
I wouldn't recommend continuosly cranking on a starter for long periods of time without giving it a break to cool down. It will get very hot and reduce the lifespan of the starter. Such abuse can easily finish off an older, worn starter. I've also seen someone melt their starter wires by doing this.
If the engine doesn't fire in the first 10-15 seconds, I'd wait 30-45 seconds (with the glow plugs on) and try again.
ANY extra knowledge about your car can potentially save you money.
Mike