I just started my cat on Heartgard

I bought six months of chewable Heartgard tablets and gave my cat her first dose today. I had to mix it in with tuna. I keep her indoors, but mosquitos manage to get in, so I thought that it would be a good idea to give her some preventive medication. When the supply runs out in December, should I buy another six months of doses right away, or wait until summer to for the next dose? It's not as if there will be any mosquitos in January and February, or even March.

I'd talk to the vet, but I just started mine on Revolution. My vet suggested I just do six months - spring to October, since we don't have mosquitos in the winter here in Chicago, and mine are indoor cats (unless they make a dash for the courtyard!)