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It depends on your purpose as to whether flats are important.
I do photometry so flats are very important regardless of the exposure time. My camera remains permanently on the scope so I don't do flats every time but reuse them.
I take them on overcast days in my observatory with the roof in place, The scope aims at a white board on the wall and hopefully the diffused light on an overcast day gives me a pretty even illumination. For R and I (infrared) filters I still need to put a T shirt over the front of the scope to get the exposure to be long enough.
You should take separate exposures through each filter if you are using filters and at different binning if you need to. I tried not doing this but if you bin the flats in the puter afterwards the gain data is incorrect in the fits header.
Try not to have too short an exposure as the shutter can give uneven exposure. 5 secs seems a good minimum.
If your setup isn't permanent then you will need to do them each time. The T shirt works well but you must make sure that the T shirt is evenly illuminated itself and that it is aiming at an evenly lit surface. The focus position must also be the same as when you are imaging.
Hopefully this is helpful.
Cheers
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