I dont think for first extraction you should do a very big extraction, just in case something goes wrong. There's a few people that do like 50g mimosa first extraction, which should result in 400mg-1g DMT depending on bark and extraction thoroughness. Though the work of making a bigger extraction isnt much more than of a small one, so I think maybe if you read it up well and feel like you undertand the reason behind the different steps of the extraction process and make sure you have clean solvent and so on, it might be worth it to make a 100-150g extraction already, and then just store the DMT.
As the FAQ says, DMT doesnt really degrade significantly. With light/air/heat exposure, it can get darker/waxy but it is still perfectly active IME, even after very long time. Seems its only a superficial oxidation, and DMT N-oxide is anyways psychoactive. In any case of course ideal is storage in some dark airtight vial/container in the freezer or fridge. Often people also do a fumarate salting of the solvent to have DMT Fumarate, since that is considered to be a very stable salt form (and one can either take it orally as it is or convert back to freebase doing the conversion mentioned at the end of BLAB tek with sodium carb and water)