It is different for everyone.  But when they feel the same way is a good clue.  And people change and during a marriage you can even feel in and out of love for one reason or another - hard times, loss, etc.  I have been married for 38 years to the same man and have never once cheated and I fully believe the same of him, it has not all been easy, but we have been willing to stick it out and there has not been one other person ever I would wanted to go through life with.  We have supported each other no matter what and have had each other's backs when necessary.