For sure, you will not get the same duration from sildenafil citrate (Viagra) or vardenafil (Levitra) - or from any of the generics for these drugs and certainly not from any supplement or “magic” pill that I know of as compared with Cialis (tadalafil).
It is all about what is called “half-life” and how your own metabolism works. Half-life refers to the time after dosage at which half the dosage you took is still working. For Viagra, the half-life is on average a little over 4 hours. Take 50 mg of V and 4 hrs later you should have, still, 25 mg working for you in your blood stream. Although the half-life of Levitra is about the same as V, in practice most men get longer runs from it than from Viagra (perhaps because because the liver breaks L down more slowly). The half-life of Cialis is, however, 17.5 hrs.
But none of this is etched in stone, so to say. I can get 4 - 5 day’s effect from Cialis, 6 - 7 hrs from Viagra, 8 - 10 hrs from Levitra. But if we are talking about quality and “oomph”, what happens on Cialis after 3 days is nothing like at one or two days.
A tip about food intake. V works best on a totally empty stomach (no food at all during 3 hrs prior to dosing) but you can get away with eating by dosing prior to a meal, getting the stuff kicked in, then eating - but not heavily. A romantic dinner in or out with Wifey, GF, BF, is possible. Levitra has less negative interaction with food intake, but that doesn’t mean you can go gorge yourself on yummy, fatty foods and still perform. Cialis works best taken on an empty stomach (they all do), but once it has kicked in for you, go eat and it’ll still work, unless you eat at Burger King or the like.