Commission Model for selling IT Consulting Services

I'm looking at exploring a range of different possibilities for a commission only model for selling IT Consulting services. This is for a company that sells buckets of Professional Services day. Normally a customer signs up for 36-48 PS days over a 3 month period, and if the project goes well they continue to buy more PS days from us. Right now I'm on a decreasing tiered model, meaning, I get 12% of the first month, 11% of the 2nd month, 10% of the 3rd month so on and so forth all the way to 0. Please note this is not a full time job.

I'm looking for opinions on if this is a favorable model in the eyes of other sales professionals or if I have room to make an argument for an indefinite residual commission model. (Thinking something like a 1% or 2% monthly residual model after the decreasing tiered model hits the floor)