If you vape DMT on harmalas, the delayed onset is slight yet significant. I typically feel the DMT coming on 5-10 seconds after inhalation (and before exhalation starts). When harmalas are thrown in the mix, the onset is delayed to somewhere typically between 15-30 seconds. Again, this is a relatively small difference, but the point is that if you're used to it coming on in 5 seconds, and you're not noticing anything take effect, you would want to probably wait at least another half a minute before taking another hit because its very possible you already have a whammy of an experience heading your way.
In terms of how long the whole thing lasts, that will depend on a variety of factors. It will obviously differ first and foremost from person to person. Strong experiences may tend to last longer than weaker ones. Duration will also be affected by how much harmalas you consume, and via what route of administration. Smoking/vaping harmala extracts will typically result in a shorter duration for an experience than if it were consumed orally. I find that strong vaped experiences on vaped harmalas can last for me upwards of 40 min. With oral harmalas, the experience can last upwards of an hour if it is very strong, and I am lucky. The afterglows may be a little extended as well. Experience duration is always kind of variable. There always remains the possibility that you'll do everything "right" with the harmalas and DMT, and still wind up with a 5-10 min experience.
In terms of how long the whole thing lasts, that will depend on a variety of factors. It will obviously differ first and foremost from person to person. Strong experiences may tend to last longer than weaker ones. Duration will also be affected by how much harmalas you consume, and via what route of administration. Smoking/vaping harmala extracts will typically result in a shorter duration for an experience than if it were consumed orally. I find that strong vaped experiences on vaped harmalas can last for me upwards of 40 min. With oral harmalas, the experience can last upwards of an hour if it is very strong, and I am lucky. The afterglows may be a little extended as well. Experience duration is always kind of variable. There always remains the possibility that you'll do everything "right" with the harmalas and DMT, and still wind up with a 5-10 min experience.