LawnGuru credits never expire! If they're on your account, and haven't been applied toward a service, they will remain there until used. However, only $15.00 can be used at a time per service. If a customer were to have $30.00 in coupons, that would break down into two $15.00 off cuts.
If you are granted a promotional coupon at the time of registration, those coupons will be applied according to the guidelines of that promotion.