Eh, i've never really timed it, but i'd have to guess ~2X long, depending on the person. My prefered method, if you don't mind going through the trouble of extracting the MAOI, is eating the DMT, and smoking the MAOI. Also works well for mushies, and cacti/mescaline. Supposedly you can inject harmine/harmaline, which would be interesting as well. Remember though, it's doesn't just increase the duration, but the intensity as well. So be safe, be prepared.
And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove that what is good, and acceptable, and perfect, will of God. Romans 12:2