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I find this depends on the field and the brightness of the star. I typically guide on another star in the field and there are usually several candidates, with my guide images of 5 to 10 seconds. However, this may depend on how good your mount is at tracking.
I use PHD2 and have guided on the target star on occasion. I’ve not found any problem with this, but there does need to be a strong signal outside of the slit. I’ve not looked in any detail as to how this works, whether it needs bimodal guiding or not. If the star is bright enough then I tend to notice it dimming rather than disappearing or splitting in two. This is usually the case when I am taking a spectra of a bright star and so the fainter stars are not easily visible in the guide image.