Yes, Viagra, Levitra, Cialis, Stendra, all are great. I ordered a batch of generic Levitra and started using them from January this year. I found them to be better than Viagra. Faster acting, lasts longer, but same side effects and costs about 20% more than Viagra of equivalent dosage.

How long it lasts depends on a lot of factors. Your body mass is the main factor. If you're a bigger guy, 100mg will probably go for a bit less than the 4hrs as written on the box. The next big factor is what you do with your body after taking it. These meds are passed out of your body mainly through poo, with a smaller percentage through pee. That means if you take diuretics and wee a lot after taking them, you will reduce the period of effectiveness.

About high doses, there's a sub-community out there that is obsessed with ultra high doses. There's a "superman combo", high dose Cialis + high doses of Viagra closer to the action. That's crazy, but you can understand why it's called "superman combo", lol!

They also did a study on an Indian farmer who regularly used 700mg Viagra and found no "issues" with his health. Go figure.