If you plan to do that, first you would need to create a free
United MileagePlus account (this is a separate account from the cc) and then link your United Airlines MileagePlus account to your United MileagePlus Explorer credit card account when you sign up. The rewards from the affiliated Explorer credit card go to the United Airlines MileagePlus account after they are earned (I think they may post once a month or so). As long as you have the Explorer credit card, the miles in your United Airlines account will not expire. If you have the United Airlines account only (without the Explorer credit card), your miles will not expire until the 18th month of account inactivity. I copied relevant info from the United website about mileage expiration for United Airline miles in case that helps.
Mileage expiration
10. Any member who fails at any time to engage in account activity for a period of eighteen (18) consecutive months is subject to termination of such member’s membership and/or forfeiture of all accrued mileage as of the last day of the 18th month. Activity includes (without limitation) earning flight miles on United or earning or redeeming miles with a MileagePlus partner (as defined in Rule 12), redeeming miles for award travel, buying miles or transferring miles.
27. Mileage accrued in a member's account shall be maintained in the account until it is redeemed for an award or until it expires, whichever occurs first.
28. Mileage accrued shall only be subject to expiration on the last day of the eighteen (18) months after the last account activity in the member’s account. Activity includes flying, using your MileagePlus credit card and more (see rule 10). In cases where mileage is for any reason removed from an account, such as for the redemption of awards, and later returned, the return of the mileage to the account shall not count as account activity.