increasing gastric pH (alkali) increases the amount of amphetamine absorbed, and the rate as well. (because less is in a charged salt form) Antacids like Tums etc will not effect duration, except inasmuch that you absorb more drug and hence it lasts longer.
increasing urinary pH (alkali) will decrease your kidneys excretion of amphetamine and encourage reabsorbtion via the bladder. In amphetamine overdose cases one common treatment is ammonium chloride to acidify the urine (increase excretion rate). I don't think antacids effect urinary pH though.