How long are written adderall rx valid?

This will vary from state to state, as each state's board of pharmacy may enact it's own rules. It may also depend on your insurance.

Generally, prescriptions are good for one year. Controlled substances must be filled within 30 days.

Note that your insurance plan may layer additional restrictions on top of those imposed by your state. For example, in New York state, medicaid requires a new prescription for a non-controlled substance to be filled within 60 days from the time it is written, or medicaid payment will be denied.