I have a windowsill shaped one I use for some things sown in small pots or seed trays, but the main thing to consider is what you do with those seedlings once they are up.
For slowing growing stuff like sweet peppers, chili peppers and aubergines, I usually fire mine up at the end of January. For tomatoes, which grow pretty fast, I wait until March, then only use it if it is still very cold. It can offer a more steady guaranteed germination rate, which can be important given the price of some seeds.
Squash, courgettes and cucumbers, I usually sow in April. They grow like triffids so no benefit to me to sow early.
All of this is due to the fact that I grow on windowsills inside, then out to an unheated greenhouse. If you have a different set-up, its worth asking questions before you sow. If you post asking about peppers say, then explain what you are planning, its pretty certain someone will be in the same boat and will be able to help