With an arundinacea strain, real world you'll be lucky to get .01%, total alkaloid, if there DMT at all, it will only be a small fraction of this .01%. DMT may not be present at all, you may end up with a bunch of beta-carbolines and/or gramine.
For TLC or reagent testing you could get away with a under 100 grams just to see if there are tryptamines present at all. For an actual extraction that might yield you a tiny of puff of alkaloid goo you are probably looking at a kilo of fresh material minimum.
Phalaris really requires proper care to maximize tryptamine content, and even then arundinacea strains are extremely low in alkaloids naturally and that's just what it is.
P. aquatica and brachystachys naturally have higher alkaloid levels and more consistent tryptamine profiles, they are a much much better bet if you really want to pursue the grasses as a tryptamine source. You're really better off planting a few acacia trees though.