Taking care of hair extensions is not that easy. We can obviously ignore our own natural growing hair, but if we want to keep extensions for a longer period they would definitely require good attention. Mostly professional suggest that extensions should be removed with in 4-5 months however, if take care of them they can last for a longer period as well and can still manage to stay in a good shape.
If you are getting synthetic hair, you need to check the quality of the hair first. The thing is that if you have gone for cheapest hair, you won't be able to experiment much with styles as they cannot afford to bear so much of heat of straightening or curling irons. A few kinds of synthetic fiber such as monofiber or thermofibre might be able to take the heat. If you have to use styling tools to your extension you need to check the quality of the hair. If they quality does not support heat, avoid using heating products.
You need to use a comb or brush specially designed for this purpose. Using any regular comb or brush will only damage your extensions so you need to be careful with that too. You can always use a detangling spray on your hair extensions so that they do not get frizzy.
Most of the hair extensions are designed in a way to stay straight. If you want to curl them, you will have to be really careful with the procedure. Avoid using curling iron and try using hair curlers or rollers. Never ever go to sleep without washing your hair properly especially if you have sued any styling product on the extensions.
Anyone would like to keep and maintain extensions for as long as possible. However, try not to go overboard, there will be a point when they will start looking ratty even if you have been taking care of them. Turn to a professional and ask him if it is the right time to get the synthetic extensions removed. You can always get extensions again if you want to.