It doesn't 'last' for a period of time, it's just giving them a clear out every so often. Our practice recommends every 3-6 months, most clients are using a monthly product for lungworm that treats most intestinal worms anyway, so the Drontal is just to cover tapeworm. If you are not using any other wormer I'd do it every 3 months.
Drontal should be given at monthly from 3 months to 6 months and then quarterly (every 3 months) after that. You should be using another product to treat fleas and lungworm. You usually have to use 2 products to make sure your dogs are covered for fleas, ticks, worms and lungworm.
If you are using Frontline and Drontal you have no lungworm protection - talk to your vet about prevalence in your area but it's a problem in most of the UK now and puppies especially vulnerable, because it's passed on by eating slugs, snails and frogs, or potentially by contact with their slime.
I was wrong. Woody is on Advocate. That still doesn't look like it covers tapeworm though. The box says Flea, larvae, whipworm, hookworm, roundworm and angiostrongylus, credo soma, heart worm, otedectes, sarcoptrs, demoted and louse.