Given that you enjoy racing, and want to do fairly well, how often do you buy new sails?

What do you replace most often (do you run your spi as long as your mainsail)?

We go sailing/training 1-3 times a week, and usually have 10-14 races in a season. Average conditions are low winds, but some really windy sessions as well, we are a heavy team.

We have found that the spi is really tired after two seasons. We would like to replace that every year if we had the budget. Mainsails and jibs seem to hold up longer (no snuffing and fully battened). We would like to replace those every second year.
We recently hoisted a new mainsail, replacing a 2000 model 1.th generation Tornado pentex square top, and added heaps of both power and speed. Next to go is our 2000 model dacron jib!

Given normal use, how fast will a pentex jib and main loose it's shape?
Would like to hear what you others think and do.

I also suspect that many sail around with the sails that came with boat until they literaly fall apart.