Manual gearboxes are very tough churchill. They don't "Go at any time". The people you spoke to were laying it on to worry you, so don't worry about that. Automatic gearboxes do suddenly fail, but manual gearboxes are pretty well bombproof and will last the lifetime of the car.
I'm guessing that the gearbox is making a graunching or singing noise as you drive along in one or all gears. If so, then one or more gears are getting worn. It'll keep going for ages, but the noise will just keep getting worse until the looks from pedestrians forces you to think about repairing it. Believe me - I've been there!
If you want to get it sorted now you can get a second-hand gearbox from a breakers. If there isn't one near you then there are many on the internet - but they do charge what they like! Failing that, there are many companies that specialise in fitting reconditioned gearboxes. You drive your car to them, wander round the shops for a few hours and drink tea until the work is done, then drive home. The best and cheapest option is to give the car or gearbox to someone who specialises in rebuilding gearboxes. You'll find ones local to you in the Yellow Pages or the Internet.
Good luck.