No -- love doesn't always present itself as a spark. If you leave as soon as the spark is gone, you miss out on something deeper that comes with long-term commitment in a relationship. Every relationship goes through its ups and downs, and you won't always feel the love -- part of growing together is learning how to keep it alive and apparent through even the toughest of times.
If the love is completely, absolutely gone from a relationship, then yes, it's probably time to move on. If all you're willing to give it is a few weeks to see if the love comes back, you'll probably regret the decision and never know what else might have been possible.