Rule of thumb, at cellar temperatures water holds about as much CO2 as the vapor space does.
So 2 atmospheres of pressure would be in equilibrium with a beer carbonated with 2 volumes of CO2.
1bar = 1 atmosphere = 15 psi (ish, close enough for brewing)
Note that there is already 1 atmosphere pressure (referred to as gauge pressure). So if you purge the air from your keg with CO2 and the gauge reads 0 this is really 1 (or 15psi-a), if it reads 15psi-g you have 2 carbonation volumes (30psi-a).
So pick your carbonation level in volumes, multiply by 15, subtract 15, and this is what you want to set the regulator to store the keg at . Speed things up by filling the keg to it's max pressure and giving it a shake to dissolve the CO2 (always purge first, otherwise the partial pressure of CO2 is only going to be a fraction of the total and it'll take far longer and oxidize your beer). Leave for a few hours with the regulator set just below the max of the keg, shake again. By this point it should be getting close, so drop the pressure on the regulator to whatever you calculated, vent off the excess and leave it.
30bar is a bit mental though. If your keg exploded at that pressure it'd take part of your house with it! 30psi would be a fizzy lager.