A lifetime of prayer cannot be recompense for God’s grace. And, what we view as a small grace actually has great and eternal consequences, which go far beyond what we may perceive. Thus, it would be inadequate to spend our lifetimes in prayer, as finite thanks cannot repay an infinite benefit.
That being said, we can (and should) include thanks in our habitual daily prayers - recognizing that an intention received is as much a blessing as that which was denied; that suffering is as much a blessing as heath; that spiritual dryness is as much a blessing as consolation.
A long way of saying that we should pray constantly with our hearts properly disposed. A simple, profound, heartfelt thanks can be completely satisfying to God. No need to add up the hours and days. God is not that type of accountant. His ledger records love.