My wife is going to UK for Postgraduate diploma in Business management(With English course).Lenght will be 20/9/2010 to 30/3/2012.
>can she work there?if yes then how many hours?
>can her dependent work there?if yes then how many hours?

Are the dependants of work permit holders allowed to stay in the United Kingdom?

This page explains if a work permit holder's dependants are allowed to enter and stay in the United Kingdom under their work permit arrangements.
Dependants

A dependent is a wife, husband, civil partner, same sex partner, unmarried partner and child under the age of 18 years old. The dependent of a work permit holder may be admitted to the United Kingdom. They will be given conditions of stay which do not place any restrictions, other than those that would apply to a resident worker, on employment or engaging in business. This normally applies when a dependent is granted more than six months leave. If they are granted less than six months leave they will not normally be allowed to take up employment. Any dependent children over 18 years old should apply on a separate form and pay the appropriate fee. If you are overseas, you should see the visa services website for further information on how to do this.

The exceptions to this are the dependants of multiple entry work permit holders who, if they wish to accompany a work permit holder to the United Kingdom, must qualify for entry in their own right under another category of the immigration rules.

Source: UK Border Agency | Are the dependants of work permit holders allowed to stay in the United Kingdom?

I am doing bsc hons in nursing studies in uk.can my husband join me and after how much time they could apply

How many hours dependents work in the uk per day