Looks like if you activate with hotspot you get a 100 dollar gift card, but I wonder how long you have to leave it on
I'd say just 60 days -- terms state:
Gift card codes will be sent via e-mail within 60 days of purchase, one per customer. Hotspot feature must be activated at time of purchase to qualify.
I'd recommend skipping the hotspot activation and just getting the phone itself -- then just follow instructions in this thread
to get Unlimited Internet for 3G/4G phones (you'll need to add hotspot to your plan to get the Unlimited Data trick to work).
Unlimited 4G data for the life of your contract is MUCH slicker than $100 g/c