I've had my current set of slime tubes for at least three years, I forget exactly. Mine are used off-road and I'm always surprised how many thorns are in the tyre when I do take it off; each and every one would have been a proper puncture in a normal tube so they've saved me many a field-repair.
I've never had any trouble patching them, just clean off the leaked slime with a damp cloth, dry thoroughly, a light rub with fine sandpaper around the puncture and patch as normal.
I only patch mine when a tube has a catastrophic puncture - last time the culprit was a nail - minor punctures self-seal. It's worth taking the tubes out now and again though to remove any sharps from the tyre casing.
One point to watch is that the valves gum up with dried slime and they become difficult to inflate. Simple asnwer with schraeder valves is to remove the core and wash it, and the valve case. Prestas are a bit more tricky, I usually squirt mine with water whilst bobbling the pin up and down - doesn't always work though and they can become permanantly leaky.
Finally, add a little water once a year, about a tablespoonful. Easy enough with a schraeder and this keeps the slime slimy
EDIT - forgot to say, I use the
Halfords own brand and have found them to be excellent.
