You can't suspend a PM account and get back credit for any subscription fees already paid. If you are on a promo or grandfathered plan, the only thing you can do to minimize your cost for the period is to remove autopay from your account and let your plan expire on its own. From the date of expiry, you can then wait up to 89 days before you must either reactivate, or lose the plan, phone number, and account. The 90th day after expiry results in account removal, so if either your phone number or plan are important to keep, I advise not waiting until the last day to reactivate, to give yourself some margin against unforeseen problems.
If you need longer than 89 days, and if you are not on a promo plan, or you are okay with resuming service on any current plan, then another option is, near the end of the 89 days, change to a text only plan ($20 for 30 days), then let that run out, wait another 89 days if desired, and repeat the last two steps for as long as necessary. When you are ready to resume service for real use, you can then choose any of the in-market plans. Be sure you really want to do this though, as any change to a promo plan results in losing that plan forever. Note: changing plans on an expired account can only be done by a PM moderator, so you would have to send them a private message to get that done.
