Whenever I look up how long it took for a blacksmith to certain tools or weapons, I just get information on forging swords. Let's say a peasant is trying to make his scythe efficient for self-defense or fighting an army and takes it to a blacksmith to temporarily turn it into a war scythe. How long would this take?
As an amateur blacksmith I can 'educated guess' based on the link in the comment provided by T.E.D.
The first five illustrations appear to show how an agricultural scythe would be converted for 'military' use. The attachment of the blade needs to be converted from horizontal to vertical since the weapon will be used for thrusting and chopping. The blade attachment would have to be similar to a swords 'tang' since there is not enough material to make a socket (which would be a better method). The third illustration shows additional reinforcement around the attachment point to strengthen it. The fourth and fifth illustrations show separating a portion of a heavy scythe blade and converting it to a forward thrusting spike. The blades from the first few illustrations are 'shallow' enough to not need this.
If you assume the blacksmith had the straight wooden pole and the additional materials for reinforcing the attachment already available the time to convert the tool would be on the order of four to six hours. The actual forging time would be at most 30 - 60 minutes for someone of a journeyman's level of experience. The majority of the time is taking the tool apart and then fitting the shaft to the blade, reinforcing it, etc.
Now a grain type scythe is not a particularly heavy bladed tool. If you tried to use a converted scythe against armored foes you wouldn't do much damage with a chop. You are better off trying to kill/injure the horse they are riding on or sticking to 'hedgehog' or phalanx type maneuvers (which farmers are not trained in so good luck).