With the specs for a silk liner made by Helsport ( a norwegian producer of sleeping bags) you can expect a comfort temp. about -16 degrees celsius. And with a fleece liner -18 deg. celsius. Thats if your sleeping bag has comfort temperature of -10 degrees.
I believe it is about the same for other brands as well.
I recomend that you buy a outer layer (gore-tex, nylon) for the sleeping bag too. Greatly improves the comfort when cold. Just remember to wring it off in the morning and shake the moisture out of it, even if it`s gore tex.