There is no set distance. I would say using basic Siamese (twin) hose (you can separate it) that comes in a 12 foot long minimum length will be sufficient.
You should have regulators and concentrator out of general work area so they are not subjected to heat from torch and glass shards and enamel or frit dust, bit not so far away that it becomes a major trip to go and make a adjustment. But close enough that you can shut thing down real fast in a emergency.
I know, this does not give you a measure able distance but each installation is different, any you have to work out a setup that fits your comfort level.