Mark 11:24
"Therefore I tell you, all that you ask for in prayer, believe that you will receive it and it shall be yours. When you stand to pray, forgive anyone against whom you have a grievance, so that your heavenly Father may in turn forgive you your transgressions.”
From this, I note that in order for prayer to be effective one must a) believe you will receive it, and b) not hold any grudges against anyone.
Note that you “will” receive it if you believe. That means you should believe even before you receive the results.
So I say if you meet both criteria, there is really no need to pray again. Doesn’t mean you shouldn’t; it just means you don’t need to. I don’t think it hurts to pray again, especially if you feel your faith is stronger or your heart is more forgiving than the last time you prayed.
As far as I’m concerned, if I were to take God at His word, then once I pray in a state of having forgiven others and belief I will receive it, then I won’t pray for it any more as far as getting it, but will pray in thanksgiving for having received it – whether or not I’ve seen the physical manifestations of that answer.
But that’s just my own opinions; your mileage may vary.
Alan