Currently, the US produces about 10% more natural gas than we consume. The Marcellus Shale play is only about 20% of that production while the rest is from other shale plays and drilling, both onshore and offshore.
Estimates are that our production should continue to increase consistent with demand, but since natural gas is non-renewable, it will run out eventually. Tough to say when, but geologists think there could be as much as 200 trillion cubic feet of gas just in the Appalachians (we consume about 30 tcf per year in the US). The problem is that we have to frack to get it, and we all know how controversial that is these days.
We can't supply all of Europe in addition to the US, but we can certainly supplement their supply for decades IMO.