30 days is normal.
To be blunt, I do not see why a 'subbie' should receive payment any sooner than the company giving him the job i.e 30 days.
Whether just being a subbie, or running a small [or large] company, there is no difference, both are running businesses and really should NOT expect payment sooner than 30 days. It is how it is and really when starting your own business that should be accepted.
We do pay 30 days, yet have contracts [That I agreed, and am more than happy with] with companies who pay 60-90 or longer!
I will get slated for this no doubt, but in my previous Business management studies, we were told that the 'clever ones' are those that hang on to their money, after all money makes money so it is better off only paying at the death, a classic company that do this is none other than BT. It is not clever paying someone before payment is due. As a previous manager with them we were not allowed to pay suppliers until the last possible minute!