Incense attracts a Pokémon about every five minutes, or at least that's the number getting thrown around. There's gotta be some RNG involved in there, but so far it hasn't varied too much for me. No idea if walking while using incense will achieve any different results.
Feel free to use the ones you get for free from levelling up, but I wouldn't buy any extra ones from the shop. Supposedly Lure Modules attract more Pokémon than incense, but I haven't used enough of the Lures to really have a feel for that.
Also, I've never gotten anything other than really common basic Pokémon from incense. Others might have different stories.
Ok so ignore this I was wrong! Listen to the other guys; it's been shown quite extensively now (and I tested this myself today) that you'll find considerably more Pokémon with Incense if you're walking around whilst using it. As people have said, the spawn rate is once per 5-6 minutes whilst stationary and once every 2-3 minutes whilst moving. Wish I'd realised this earlier - oh boy did I waste a lot of PokéCoins there...Incense isn't great basically. It's a sure-fire way to get 5-6 Pokémon appear but it'll usually be around 4 Rattata/Pidgey with 2 uncommon Pokémon throw in (think Magikarp, Poliwag, Psyduck, etc). It isn't bad if you've got it for levelling up or something but it isn't something I'd really suggest spending money on - I bought 25 of these in the first week with PokéCoins and boy do I regret it!