How long will the new I-90 be valid?

Congratulations! Can you tell me that when the new I-94 will expire? I plan to go back to China also. I have to stay there for about 5 months. (I'm holding H-1B.) If I go to Mexico to get the visa before I leave US, will that visa still be valid 5 months later when I return to US?
Also which city in Mexico did you go? Really appreciated if you can share more info!
Have a good day!