It really depends on the brand of contact lenses, I assume your talking about the fashion contact lenses, with colours or patterns on not contacts for correcting vision.
I've worn coloured contacts for a while now, they used to not last very long, about 30days max but they do seem to be getting better.
If you've never worn them before you'll probably find them a bit strange and uncomfortable for about an hour but after that you'll probably forget you've even got them in, except people will comment on them loads! :-)
You shouldn't wear them constantly anyway, they say on mine you should only wear them for 8 hours a day, although I often wear mine longer than this and don't have any problems, you should take them out when you go to bed, or if they start getting uncomfortable (scratchy) during the day, so keep the little pot or a contacts container with you so you can take them out and not lose them, when your not wearing them you have to look after them, keep them in solution, or else they will dry out and become brittle and too itchy to wear.
Just wearing mine during the day (and sometimes not wearing them) I can make them last for three months, But my friends who wear them have found they can still only get theirs to last a month or two before they become too itchy to wear.
They should cost between £20-40 unless you order online. you can find plain coloured ones for as little as £7 but be careful of quality. you may find that one companies lenses are nice and comfortable and anothers, even if more expensive, may be itchy and painful and no good for your eyes, just be prepared to be disappointed as sometimes it happens to me still and i've been wearing them for years.
Make sure you buy from a decent company, and if your eyes become red or itchy TAKE THEM OUT! you can always buy a new pair of contacts but you only have one set of eyes, be careful with them and look after them, enjoy your new contacts.
-Always buy good lenses (expensive isn't always best, ask around what other people wear and try a couple of brands till you find the best ones for you.
-Take them out if your eyes become red or itchy, and before you go to bed.
-Keep them in solution when their not in and they should last the stated time if you look after them.
do these things and they should last as long as they say or longer.