Below is a general guideline for how long it takes each type of vegetable crop to be ready to transplant once sown from seeds. These timings are based on ideal seed sowing conditions and fertilizing once the first set of true leaves have grown.
If it is too cold outside, then sow indoors. If sowing anything in the brassicas family, lettuces, and other greens like spinach during hot weather, then sow indoors as they are easily stunted if exposed to high heat.
If its rainy or the disease/pest pressure is very high at the time of sowing seeds - then consider sowing seeds in a covered/screened patio to protect seedlings.
Tomatoes - 8 weeks
Peppers & Eggplants (slow growing) - 12 weeks
Onions & Leeks (slow growing) - 12 weeks
Cucumbers & Melons - 4-6 weeks
Flowers - 4-6 weeks
Broccoli, Kale, Cabbage, Brussels Sprouts - 8-12 weeks
Small greens like lettuce, tatsoi, bok choy, spinach - 4-6 weeks
Squash & Pumpkins - 4-6 weeks
Corn & Beans - Seeds are normally direct sown but you can sow seeds in 72 cell seed trays if needed. They will be ready to transplant in 3 weeks.
Herbs - 6-8 weeks