It would depend on your tolerance. If you are completely new to the drug, then 10 - 20 mg will actually be enough. With some tolerance, users will go up to about 60 mg for a euphoric effect, and can go up to 100 - 200 mg within a 24 hour period when tolerance raises even more. Some may find themselves reaching 300 mg, but I probably never surpassed 250 mg in a day during my highest dosing habit.
Many will say that the ceiling of Adderall dosing is 60 mg, meaning that taking any more than that in a single dose is either wasteful or will cause unwanted, maybe even dysphoric, effects. The anxiety and body tension during the comedown of a high dose can make you feel like you are going to die. A benzo or an opiate can eliminate this, but (especially the benzo) can also make you feel depressed.
One problem I ran into involved raising my dose often when I only assumed my tolerance was higher. This didn't cause bad effects really, but unnecessarily raised my tolerance even more. Abstaining from the drug for several days, and never taking it for more than two days consecutively will actually help keep tolerance down. If you have never used Adderall, do yourself a favor and don't take it everyday, or you will develop a permanent tolerance to certain aspects of the drug's effects.
