Floral stretch or light stretch?
Plants will stretch reaching for light if the lamp is too high or if there is too much red in the spectrum. Use bulbs more in the blue spectrum AKA halides during vegetative. Having them in wide pots also helps slow vertical stretch. I am growing in huge 27 gallon totes, thus they have a very wide root space, thus they grow horizontally faster than vertically in these units during vegetative stage. Short and wide.
They will also stretch during the budding stage due to floral hormones. The general rule of thumb is they stretch for 40% of the flowering cycle. Thus a 7 1/2 week flowering strain will stretch for 40% of that time, or three weeks. If you know how many weeks they flower, you can easily estimate how long they will stretch.
Eg, a 9 week flowering strain can be calculated with this simple formula:
9/100 = x/40
9x40 = 360 (9 weeks flowering time multiplied by 40% stretch = 360)
360/100 = 3.6 thus a 9 week strain stretches for 3.6 weeks. (result of previous calculation, 360, divided by 100 gives us stretch time)
Also, keep in mind that sativa strains stretch much more than indica strains. This is why indica based strains are so popular with indoor growers.
Topping your plants will also slow the vertical growth of plants.