Windows 2003 native mode. One domain controller on-site, read-write, no RODCs. DNS is running on the DC onsite. Servers would be pointing to the DC for DNS and authentication.
How long would the child domain remain functional without it's ability to contact the forest level domain controllers? Could users still log in, up to several weeks after the disconnect, or would they experience Kerberos authentication issues?
Is this server a Global Catalog server? Is it the only domain controller in the child domain?
If those two are yes, it can be left like that for a while without any adverse effects. However, if it is a no to either than you could see issues really quick.