Usually you need at least a checkout (1-2 hours), but if you haven't flown in the US at all, it's definitely worth to fly some time with an instructor. It's not about piloting only. The airspace is different, the controllers are different, even the phraseology is somewhat different. For example, for me it initially was hard to understand the ATC. After some time you get used to it, but initially it is very frustrating. But the experience is really great. A lot of traffic, no landing fees almost everywhere, much cheaper flight hour, great infrastructure, great flight planning and weather briefing resources (for free!), everybody speaks English
