as a rule of thumb, i wait for the scion to flush out one set of leaves, and then harden off, then flush out a second set...(and I usually unwrap before it hardens off a second set)
but to be safe, just wait as long as possible, and keep an eye on the graft union, making sure the tape isn't girdling the cambium....this can take over 3 months in some cases...and I have seen the tape left on for a year without any problems (but this all depends on the type of tape, and the plant being grafted)