Dictionary.com says to use “any time” in this context. The difference between “anytime” and “any time” is that “anytime” is an adverb, and “any time” is a noun phrase. So “any time” follows a preposition, as in “You can call me at any time”, and “anytime” modifies a verb, as in “You can call me anytime”. However it is also appropriate to use the noun “any time” instead of “anytime”, such as …
