Butyl pond liners
A butyl pond liner will last a long time if cared for correctly (that's not hard) and fitted correctly (that's not hard either).
Tips to increase the life of your liner
A pond liner is tough, but it can be torn, so ensuring there are no sharp roots, twigs or stones protruding anywhere from the base or sides of your hole is essential.
You can buy a felt covering to use on your hole before putting the butyl in, but an old carpet works just as well, and you can probably get one or more for nothing from friends or the internet (freecycle type sites). Line the bottom and sides of the hole with carpet.
Ensure the water level comes up to the coping stones (stones around the top) - this will prevent frost / sun damage to exposed butyl. Again, butyl is TOUGH. Years in the sun and it will still be OK, but why not make the best effort to keep it out of the sun and have that liner last a lifetime.
Good luck with your pond