No fixed number of miles applies to every situation, since it depends on how severe the leak is and how closely you monitor your fluid level. A slow leak can often be driven on for a short while if you check and top off the gear oil regularly, but the safest approach is to get it repaired as soon as you reasonably can.
The real risk with a pinion seal leak is not the leak itself but what happens once the fluid level inside your differential drops too low. Gear oil does two important jobs: it lubricates the gears so they can turn smoothly, and it carries heat away from them as they work. When the level gets low, the gears and bearings start to run dry, increasing friction, heat, and wear. That is when a cheap seal replacement can turn into a much larger repair involving the internal gears themselves.
If you are dealing with a leak and cannot get in for service right away, checking your differential fluid level periodically and topping it off with the correct gear oil can buy you some time. Keep an eye on how quickly the level drops, since a worsening leak means you are on borrowed time before it becomes a bigger problem. You should also pay attention to any new noises, such as whining, humming, or grinding, since these suggest the gears are already being affected, and driving should be minimized until the vehicle is inspected.
How much time you have also depends on where you drive. Long highway trips and heavy loads put more stress and heat on your differential than short trips around town, so if you know your Jeep differential is leaking, it is wise to avoid towing or long drives until it is fixed. Cold weather can also make an existing leak seem to slow down temporarily since fluids thicken in the cold, which can give a false sense that the problem has resolved itself when it has not.
Ultimately, treating a pinion seal leak as something to schedule around rather than something to fix immediately puts your differential at risk. The repair itself is usually straightforward when caught early, so there is little upside to waiting.
If you are unsure how urgent your particular leak is, Hoover Street Auto Repair in Ann Arbor can take a look and give you a straight answer about how much time you realistically have, backed by their dedicated Jeep repair experience.