ERC20 is an interface and not a contract definition. And since gas is paid for each executed machine instruction, it is not possible to specify precisely how much a call to transfer costs for an ERC20 contract. This will depend on how the ERC20 contract is implemented (in Solidity) and how it is compiled to EVM code.
However: is it possible to say something general about this gas costs or does there exists a reference implementation where we can precisely say how much such a method call costs? And does there exist a lower limit for this number?
By looking a ERC20 transfer calls on etherscan.io, it seems that the gas cost is between 50.000 and 105.000.
I have set the gas limit to 200.000 on a wallet that I have built. So far, I have not encountered an ERC20 transfer call that has exceeded this limit.