It depends on what you are planning to grow as to how soon to start them. Slow growing vegetables (and some that can be planted outside earlier in the season) may need to be started up to 14 weeks before the last expected spring frost, while quicker ones can be planted as little as 3 weeks before that.
I usually start hot peppers 12 weeks before, eggplant and sweet peppers at 10 weeks, tomatoes at 8 weeks. When the packets recommend 6 to 8 weeks before, I prefer to start them at 8 weeks instead of 6.
I posted a list of the start times for many commonly grown veggies under the Gardening Advice forum under "gardening tip of the week", in case you want to check that out.
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