How quickly must ticks be removed?

As soon as possible, but disease transmission doesn't happen right away.

When the tick bites an infected mouse, lyme disease organisms (coiled bacteria called spirochetes) move to the tick's intestine and go dormant. Spirochetes can live this way for years waiting for a new host. Only when the spirochetes detect the fresh blood do they wake up and begin to multiply. The tick's gut is soon packed with spirochetes (which is why you don't want to squash that tick). Finally, the spirochetes move to the tick's salivary glands and into the new host's bloodstream.

This process, multiplication and migration to the salivary glands, takes time at least twenty four hours and often longer, so rapid removal of ticks should offer protection.

Do products that slowly kill the tick and allow it to remain in place (like Frontline) prevent Lyme disease transmission? I don't think anyone knows for sure. Our preference is to remove ticks right away if possible and to use Preventic collars, which cause ticks to detach and drop off within a couple of hours. And of course, you should vaccinate your dog for Lyme disease.