I typically like to charge a % of spend on paid search accounts; however, sometimes this can be hard given larger budgets. Due to this, I also like to price based on successes whether it's increasing traffic, CTR or helping you get closer to your overall goal (sign ups, sales, etc.). This is negotiable. I selected 90 days because I think it takes about 3 months to see the true successes of a paid search campaign and gives time for the optimizations to kick in once key learnings have been obtained.