You can contribute a lifetime maximum of $50,000 per beneficiary to an RESP. The amount of annual contribution room that is eligible for the Canadian Education Savings Grant (CESG) is $2,500. You are welcome to contribute more, but the 20% grant is only matched by the government up to $2,500 per year.
Your contribution room is accrued each year starting in 2007 or the year the child was born, whichever is later. The contribution room keeps accruing up to and including the year the child turns 17.
To maximize the CESG, you will want to contribute $2,500 per year per beneficiary for 14 years, and then top it off with an extra $1,000 in the 15th year. This is because the total CESG a child can receive is $7,200. If you missed a year or started late, you can contribute more than $2,500 to retroactively claim grants. You are eligible to receive an additional $500 per year in CESG if you missed previous years set of grants. In short, you can catch up for one previous year at a time by contributing more than $2,500 per year.