[Brother Paidar]

Roughly how long does an average Warp space jump take? Is it random?

[Lomendil]

Generally speaking it depends on how far away the real-space destination is. However, the Warp is unpredictable and journeys can take more or less time than expected due to the strange time-distortions it causes. In rare extreme cases, ships have been known to emerge centuries after entering the Warp, with only a few weeks or months having passed for the ship and crew while in the Warp.

[Wargamer]

Canon fluff also suggests a general time lag for Warp Ships.

For example, a ship may make a Warp Jump that lasts a month, but for the crew, only three weeks have passed.

Then, of course, you get the truly screwy ones like Grey Area pointed out. There are a few sources which vaguely suggest it is possible to enter the warp and exit before you originally entered!

Gothic tends to support this theory with Celestial Phenomina such as the Temporal Rift...

[ForbiddenKnowledge]

The only constant warp travel speed is the Tau, they are slower than the average Imperial speed by a factor of 5 I believe. Think of warp travel like sailing the Atlantic, could be quick and nice weather, but there could be a storm that blows you off course of you could be becalmed and have to wait weeks.

[RedMagnus]

It depends how the Warp is behaving a the moment. In realtime warp travel can take anywhere from seconds to centuries, no matter the distance, although it is somewhat a factor. Shiptime is as random as the time the actual voyage takes; a minute may go by in a century or a century in a minute. The Warp is a dangerous and mutable place.

All is dust,
Magnus the Red

[Restayvien]

I suggest avoiding it altogether, but us Eldar have that luxury.
The webway's a bit like a really nice holiday. ;D