It depends on how many plants you plan on putting in and how much room you have but in the broccoli case you could allocate enough room for all the plants, buy a punnet (usually 6-8 plants) and plant them, leaving room for the rest later. Then a month later (more or less, not an exact science) plant another punnet out next to the first batch and then again a month after that. If you plant your first punnet at the very start of the season and the weather is kind you should have a longer season and broccoli ready for harvest in a staggered fashion.

Hope that helps. Can be done with other veg as well.