as per FCOM PRO-ABN-70 p62/80....cranking is stopped when "burning" has stopped. Likely 4 mins is more than enough but it seems the answer lies in the airbus iae manual...it states
8.2.2 Dry Cracking
A. Requirement
(1) A dry motoring of the engine will be needed when:
· it is necessary to eliminate any fuel accumulated in the
combustion chamber
· a leak ckeck of engine systems is needed.
To perform this operation, the starter is engaged and the engine is
motored but the HP fuel shut off valve remains closed and both
ignition systems are OFF. An engine dry motoring can be performed
for a maximum of three consecutive cycles (2 of 2 minutes and 1 of 1
minute with a cooling period of 15 seconds between each cycles).
After three cycles or 4 minutes of continuous cranking, stop for a
cooling period of 30 minutes.
So its the "maximum" limit whether the fire is out or not...either 3 cycles consecutively or continuously 4 mins then STOP! else you might risk starter fault.. anyways after tail pipe fire i would be calling maintenance not attempting another start.