Exactly how fast does anahydrous acetone absorb moisture from the air?
i have some acetone dried with MgSO4 in a jar, but i dont want any MgSO4 in my dmt fumarate.

I have a small glass funnel plugged with cotton to use as a filter, but for ~70ml acetone to filter it takes around 15minutes.

Should i be worried about my acetone absorbing moisture from the air in the time it takes to filter & screwing up my yeilds/purity/ precipitation?

Unless you are doing it outside while it is raining, it is probably not going to cause a great problem.

In case of doubt, you could put the flask you are filtering into and the funnel in a cupboard and close the door while the acetone is being filtered. That would limit the amount of water available to be sucked up while filtering. It would also conveniently limit the acetone stench a bit.

Maybe you can just carefully decant most of the acetone from the drying agent?

If the acetone is percolating through the drying agent it shouldn't be too much of a problem. Some kind of small, fairly airtight enclosure is a rather good idea there, pitubo.