It usually depends on a variety of reasons. If the player comes up with a really cool idea for a potion, then I would not even have him roll, because if he fails the roll, the cool idea goes away, which would not really be a win for the game as a whole. If it is an ok idea, I might have him roll to see if the potion he has on hand is exactly what he wanted or if it might have any side effects. For example Harry's "blending in" potion in FM would be such a case. He was blending in perfectly, but when he needed someone to pay him attention, he was blending in too perfectly. And for a rather boring idea, I would simply try to get the group involved to come up with something a little more... fuego!
Then, of course, there is what JDK says about how specific a potion is. But I wouldn't let that stand in my way too much. If it isn't something that really applies to this one and only this one occasion and would have no use anywhere else whatsoever, I might veto, otherwise, I'll let it go on the rule of cool. remember that the potion slots only refresh during a milestone, so once they are spent, they won't come up for a while. I would factor in how specific the potion the player wants is into the "roll on coolness" I described above. He might have a potion that fits the problem at hand, but it is a potion that has a much broader use and will therefore have a side effect.
A difficulty of 3 is a good starting point, I think.