20 players average. 10% rake. what sort of ROI can you expect?

how do you even work roi out? is it just a matter of playing a number of tournaments?

I'm not looking at hey if i win this tournament i win $800. I'm looking at I invest $100 and i plan to get $20 back. Hardly seems worth it though six hours to win $20. thats if i win with a 20% roi.

the only reason to play these low buyin tournies is practice for the higher buyin tournaments. $10,000 at 10% is now $1000 a tourny.

If you want to play them to make a real profit you shouldn't just beat these donkaments, you should obliterate them. Also you don't always play 6 hours unless you somehow never bust in the first hour.

20% ROI is probably about right. With only 20 players, it's not much more than a sitngo. (So you wouldn't be playing for 6 hours very often, or ever. I'm not sure how long a 20-player tourney would normally take).
To have high potential ROIs, you need large fields that are full of dead money. If the field is small, your tiny edge over a handful of players isn't going to add up to much. To put it another way, you'll (obviously) have a higher potential ROI if you have a skill edge over 750 out of 1000 players, than when you have a skill edge over 15 out of 20.
The hourly earn could be similar in two games, however, as the large MTT might take several days to complete. (And bubbling one of those after 16 hours is more painful than bubbling a 2-table event before dinner-break, as you could just reg for another one).

1. These low stakes tournaments are amazingly high variance. You need a large sample size to get a consistent ROI
2. While the donkaments are good practice for some poker skills, a <$100 small field tournament plays very, very differently than a >$1,000 large event.
3. When calculating ROI on these tournaments, unless you are just doing it as a tracking metric to measure your progress, be sure to calculate in any fixed costs (gas to and from the casino, any food or drink that you are going to buy).
4. To your point, ROI% may not be the best measure. Hourly win rate would be a better measure.

6 hours is not correct unless the blinds go up very slowly.

Oh yeah $100 live donkaments are in no way solid practice for $10ks. Not sure how you expect a positive return on those right away but let's put that aside. You're better off practicing $50 online donkaments because the gameflow is much more relatable.

I've played a lot of tourneys with <30 players online. Return on Investment stat is basically only useful for landing stakes when you're a proven winner over a decent sample. Instead of worrying about roi, you should memorize opening ranges for different ante structures. That should take a while.

Then get really into 3 and 4 betting range construction so you can have a plan for many different preflop situations.

Then study big blind play vs a single raiser.

Then study all around postflop play both heads up and multiway.

That's all good stuff to do listed above, but I'd like to talk about the elephant in the room, yup, the Ice Cream Man(ICM). The closer the decision in front of you is, and the more likely you are to be eliminated this hand or soon after, the more likely you should be to fold when facing a bet or raise. This means folding +chipev hands. These would be hands that you would play one way or another in a cash game, but in a tourney, that close preflop +chipev decision becomes a fold because of the chance of elimination. Maybe that ok draw commits too much of your stack for minimal chipev. Maybe there's a big bet on the river that you weren't expecting to see and you can only beat a bluff. Maybe you pass on that thin value bet because it commits too much of your stack. These are all evidence of the fact that investing tourney chips is not the same as investing cash game chips.

Finally, there's something to be said about playing in the money. It's important to understand that after the bubble pops, all players are now playing an exclusively positive sum game. Also, you should have some pretty good reads in such small field tourneys that you can use to increase profits by exploiting your opponents. Of course, your opponents are hoping to do the same to you, which is why tourney endgame play is so much fun.