If she is feeding well through the day and taking from you what she needs, I certainly would no need to be concerned about dehydration.
The families I work with, providing baby is gaining weight & feeding well I advise them to allow baby to sleep up to **6hrs from the start of the last night feed.
ie- if the last feed was at 9.30pm, I set the alarm for 3.30am for the next feed. I then start the day again at 7am.
She may well wake before this - in which case feed her then and time the 6hrs from then, or when 7am arrives.
She will not dehydrate assuming she, as said above, is gaining weight/feeding well.
The important thing here is that you do feed her simply to keep the breast milk supply stimulated.
If she slept past 3.30 you could express 30mls from each breast-which stimulates the breasts, essential in the first few weeks to establish the supply, and go back to sleep yourself until she wakes you.
With breast feeding mothers I work to the following times in the first few weeks -
7am, 10am, 1pm, 4pm, 6pm, 9.30pm
then not later than 2.30am (up to age 2wks & before regaining BW)
then not later than 3.30am (**after 2wks & regained BW)
Hope this doesn't sound like double dutch...