When you can hold the circle visualization. It doesn't take time to work. When a witch casts their circle, they should trust in themselves. If you don't think something's working, it isn't :P
to add to the above, the area should feel different to you somehow. i apparently cast a very strong circle, though i never feel it, my one friend can. the circle will be warm and we won't feel wind as bad. the second the circle is broken or opened, the temperature drops very rapidly, so that's one sign. you should also feel safe and secure, possibly even energized.
Wolf, that's why it's important to tune into energy. Both your own and the energy of your immediate surroundings. If you're attuned, the energy will tell you whether your circle is strong or not. Please remember that a circle is meant to protect you spiritually, no matter how strong it is. It's created to separate you from other energies, so that you can do your spiritual work undisturbed. It is also a way to create a sacred space both physically and in your mind. Having said that, the circle is not bullet proof. And the last thing needed by magic communities, is some silly person testing the strength of a circle in front of a gunman, without wearing a bullet proof jacket.
there have been times where i'll start casting the circle only to stop and start again because i lost my concentration. other times if i cast the circle and it isn't strong enough or i want it to be stronger, i'll go around three times. third option, if you're in a coven, you can get three people to go around with you. sometimes in my coven, we'll have one in from calling forth the watchtowers and lighting the candles, the second directing the energy between them and a third ringing a bell to purify the circle. do what you feel is best, but you can go around as many times as you like until you feel it's strong enough.