Request for Test Translation‏

Quite Frequently, I have been asked to provide a sample of my translation services to measure my competency. A couple of clients offered to pay for such effort. I did invoiced them however and am glad that i did got paid, which resulted in me getting further jobs.

My question is if I'm asked to do a test/sample translation, what is the magnitude of work that is considered sample? a couple paragraphs, and a lines here and there would be fine, but how do you find balance when a client needs his entire first job to be a test job? I believe that a maximum of two paragraphs will be a good starting point, whereby client can judge your ability, use of grammar and linguistic skills.  Shall there be more than that,  I would definitely would like to be remunerated accordingly.