Where do you guys recommend actually making your purchase? Also do your colognes last you a while?
Personally I like to buy from the store if I have time to hit the mall, or the store's website, so Macy's, Nordstrom, Neiman Marcus, or wherever, Sephora.
There are discounter sites where you can get fragrances for cheaper than in stores. I like to buy from the store direct because I want to feel 100% assured I'm buying the real deal. That's just a quirk on my part, it's not to say that discounters are selling fake stuff, but I have read experiences from discounters about getting older stock and issues with the scents possibly due to storage conditions. A lot of people use discounter sites, but I can't speak about the experience personally. You can definitely save a lot of money.
------------------------------------------------
Depends on the cologne - what I mean by that is, if you have a 'signature scent' cologne, or a 'daily driver' cologne and you wear it often, you're probably better off buying the largest available size which is usually 100ml or maybe sometimes 150ml for designer fragrances, I.E. Chanel, YSL, Dior, etc.
If you're talking about colognes that are considered by some to be more 'special occasion' formal scents, or seasonal only scents, (I.E. MFK Grand Soir) then yeah they can last you a long, long while. In my case, I spray my Grand Soir whenever, I don't hold off using bourgeoisie fragrances for daily wear. Gallagher's Wicked Good 50ml will probably last me 2-3 years+ because it's a cold weather scent. Day Three's BANI, same thing, but summer scent. ...whereas I burned through my 50ml of Chanel Allure Homme Sport because it's a year rounder and will have to get a new bottle soon, will get the bigger bottle next time.
Another couple factors to consider is how much you like to spray and how strong the scent is. For instance, some people only like to spray twice for a certain fragrance, where as some like to spray four times. If you only need to spray twice and the scent lasts all day for you, then you will get more mileage out of your bottle.