Looks like it's going to be 20 months for now.
https://manage.sprintpcs.com/specialoffers/RebateWelcome.doUpgrade and save $150
Thank you for continuing to choose Sprint. To show our appreciation, we offer savings of $150, plus additional discounts on our most innovative devices. All with the same quality Sprint service with lightning fast speeds on the Now Network.
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Here's how it works:
Save on a hot new device, depending on how long it’s been since your last device discount.
Customers who have NOT upgraded since 10/2/11:
Customers who HAVE upgraded since 10/2/11:
To take advantage of this offer, you’ll need:
An account in good standing with no past due unpaid balances
2 year Service Agreement
$39.99/mo. or higher plan***
After your next upgrade on or after 10/2/11, it could take 1-3 months for your upgrade eligibility to reflect the new 20 month program.
Offer details:
*An activation fee of up to $36 per upgrade may apply.
**Get $150 off OR Sprint’s nationally advertised price on a new device, whichever is better. Nationally advertised price reflects regular price minus all eligible mail-in and instant rebates.
***The primary line on the account may be required to activate on a service plan of $39.99 or higher ($39.99 is base monthly plan price and does not include fees for add-on features and services). Excludes Sprint Relay, SERO and Sprint Business Advantage Talk 200 plan. Some other exceptions apply.
Sprint may change or terminate this program at any time at Sprint’s sole discretion.