As many times as time allows! Like any other cooking system, love needs to be fueled constantly. Husbands need to hear from their wives constantly how they are love and the more he gets this either verbally or by letter, the more he loves the wife.
Its a common saying the love begets love. If the wife continues to write to her husband, even the less loving husband will return that love to the wife with time.
Surprisingly during dating the two do not want to stop communicating or leave each other but after many marriages have started things change. I think love needs constant rekindling!!!