It depends on you and your vine. Weak vine will need more...obviously.
30g is always a good starting dose for Caapi (full MAO inhibition). Too much and you will wish you were dead...seriously, it's grueling. Start slow and see how your body feels. The great thing with Caapi is that you can stack your doses. I always brew for 2/person just in case it's weak or someone want to "run up the side of a volcano, screaming at the top of his lungs, to greet god". There is no tolerance buildup, no time frame you have to follow, drink it, sip it, chug it.
Also you can get powdered Caapi, which I have taken frequently as a Toss-and-Wash (toss the powder on tongue and wash it down w/a drink)
And yes, even small amounts of Caapi will change the experience (usually for the better imho)
Changa, which is traditionally shredded Caapi leaf enhanced with DMT, is also slightly different from a pure DMT voyage.