How long can you go with a phono cable?

This is the most sensitive cable in the system for the reason you mentioned--very low signal. I think most agree that cables do make a difference, and in my experience this cable it's a difference x2 or more. Shorter is better which is why most are 1 to 1.5 m, but also the quality of the cable is very important, and price goes up with length. Thus if you can get by with a 1 or 1.5 meter you should. The cable length from a phono pre to the pre amp is much less critical. I have my phono pre close to the turntable, but then run a 2.5 meter cable to the pre-amp.

I agree with Rives, and would like to add that any cable especially phono are likely victims of RFI and EFI. This will be dependant on your system and enviroment. But I thought it might be a consideration in your decision.

The industry standard is 1.25 meters in length, although we have made cables up to 2 meters in length that have worked very well in this application. As a matter of fact we once had a gentleman who needed a 3 meter run and we were just as supprised as im at how good it worked and sounded.

Until it finds out that you've been also making good music with a 0.5m orchid digital w/XLR ends!

I agree with Rives and Theo. Don't forget about shielding, very important on a phono cable