Bark will slip at anytime once we are into the growing season. However, as Ed pointed out the bud you are grafting lies between this years current leaf stem and the bark. If you harvest it to soon it will not grow as it has not developed enough. Another thing for you to consider is that if you T-bud too soon then that bud you graft may actually have time to heal and grow this season. That new growth may not have time to harden off before winter.