You will have to look at the expiry date on her visa and you will have to apply for ILR (Indefinate Leave to Remain) before their visas expire, but you mustn't apply too early. They have to have been living in the UK for 2 years to qualify and as long as you get the application in time (ie,before it expires) and so that by the time they recieve it she will have been here for at least 2 years, you'll be OK. If you send it too early they may reject it. They don't make it easy. Once they have ILR, the next step is British citizenship and British passports which is when they have lived in the UK for three years. After that your finished dealing with the bureaucrats at the UK Border Agency and at last you can relax.
Iain.