Lake Powell was at its present level in the late 1960s, when it was initially filling; from there it took another decade to completely fill.
That means from here it would take a decade of “normal” precipitation to refill.
But that would leave Lake Mead still half-full.
Refilling Lake Mead would take another decade (though in practice, both would be gradually filled simultaneously).
That’s assuming the precipitation patterns of the last half-century return, which is by no means a sure thing.
But, about 20 years ago in Arizona I saw a single winter storm erase the accumulated water deficit of a two-year drought. Another overall winter like 1982-1983 would help a lot.
That was one hellacious year for rainfall!