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Good question!
For transmission gratings working "slitless" the resolution is 100% controlled by the seeing disk. The better the seeing the better the results.
For serious slit spectroscopy we use a reflective slit plate and guide on the slit gap/ target star. With good guiding (PHD2 or AstroArt) it's possible to hold a target star image on the 20 micron slit gap almost indefinitely - ten minute subs are not unusual. Total exposures can be up to two hours on faint targets.
The key is to get enough signal to give a SNR >100.
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