Generally speaking, the allowance is much the same as on domestic flights - in the UK it is basically a single piece for which the maximum weight may vary depending on carrier but the maximum dimensions are fixed. Watch out if you are transiting through an EU/EEA airport en-route because you will very likely not be allowed to exceed any aspect of the allowances so duty free exceeding these would be confiscated. However, returning from a non-european airport without transit through a european one, you will find allowances may be much more lax, depending on the carrier.