I leased a vehicle for 39 months. At the time they told me I was allowed 12,000 miles a year. On the contract I and the dealer signed where it says not to exceed a certian amount of miles or .25 per mile over they left it blank. Is that to my advantage???
You should still proceed carefully. I understand the paper you signed didn't have the mileage limitation on, but:
There's a strong chance they're storing it electronically, which could catch you out.
The form you're looking at might be your copy, so the mileage limitation didn't need to be written on there.
There's also a small chance that this was left blank and then filled in after you drove off the forecourt.
That last point would be pretty crappy of the dealership, but it seems odd that the dealer signed the form with the mileage limit blank (maybe an honest mistake), but it's easy / extra revenue for them.
When you're coming towards the end of the lease, the dealership will contact you, and you'll be reminded your mileage limitation and fees.
Alternatively, if you have a few more years left and want to make the most of the mileage, then you can call them up and ask to be reminded.