Take a quick look at Alex Rodriguez's first half splits:
April: .876 OPS, 5 HR, 13 R, 13 RBI May: .941 OPS, 6 HR, 15 R, 14 RBI June: .860 OPS, 4 HR, 15 R, 18 RBI July: .909 OPS, 3 HR, 5 R, 6 RBI
Obviously those July numbers are from less than 2 weeks' worth of games, but extrapolate that out over a full month and A-Rod is right on the pace he's been all season. There were concerns about his ability to stay healthy and play every day, and he's proven those wrong. There were concerns about his ability to produce after so much time off, and he's proven those wrong. There were concerns about how long he could maintain such a high level of production, and through the first half of the season he has proven those wrong. The batting averages and BB/K rates have fluctuated from month to month, but overall there hasn't been any kind of dip or drop-off in A-Rod's production. He's been a high-power/high-on base player since Opening Day, one of the best in baseball.
So how long is this going to keep up? At this point, is there any reason to think it won't? Alex is perfectly healthy as far as we know, his body and performance has held up incredibly well over long stretches of games, and he's had the benefit of short breaks here and there when the team goes on the road in interleague play. The current All-Star break is giving him another extra period of rest, and if previous periods this season are a good indicator then we could be in for some early fireworks right after the break ends.
He hasn't been perfect. There have been a few ugly games, he does swing and miss at a lot of offspeed stuff, and he still isn't doing much in day games. But Alex Rodriguez has been one of the best offensive players in baseball in the first half and he's done it consistently from game 1 to game 88. As long as he doesn't suffer any physical problems, I think there's reason to believe he can keep this up in the second half.