I dont know how old your lo is, but I would think it would depend on the normal length/time of nap. The couple times this has happened to us, I've mananged to keep dd up until about 30-60 minutes before normal bedtime, sometimes with a long winddown time in the evening (go for a walk or read extra stories)--seems to keep her from getting too stimulated but still keeps bedtime in the same neighborhood. Although we haven't really had an exact bedtime anyway, so its varied regularly since she wakes at different times each morning.
~Kim