Is it legal to be paid so late?

I am a technical architect and collaborate with an architectural studio preparing reports, as they have many clients and when they are overwhelmed by the number of projects, they refer them to me. When this happens, I always do the job and comply with the deadline that they establish, but then when I send the invoice to them, they do not pay until two months later, which causes a lot of problems to me.

Is it legal that they pay so late?

It is necessary to verify the collaboration contract that you signed with them. If there was no contract, the payment should be done when you deliver the work and the invoice. If there is a contract, then it is necessary to check the clauses established, as the time of payment is something that the parties can freely agree.