Well where do I start matey, You do understand that your gearbox is at least 12 years old or up to 16 years old on the oldest pulsars, So its already lasted quite a while wouldn't you say lol.
How much longer will it last ? answer how long is a piece of string lol, By now all the bearing's and syncro's must be pretty warn so it could do you another 10 miles or if your lucky another 20k who knows matey.
You have to remember that nothing lasts for ever on any car weather its a pulsar or an evo/skyline , Even my £3000 quaife gearbox needed rebuilding after 4 years and 36k of hard use.
Rob