It really depends on how long they have been with you, and also what your contract of employment says. There are statutory minimum notice periods as follows; 

• one week’s notice for employees with one month to two years of service
• one week for each full year of service, up to a maximum of 12 weeks, for those with two years or more. 

 Therefore, if someone has 3 years’ service, they have to give you three weeks’ notice.  

However, your contract of employment may specify a longer notice period. Always refer to the employment contract for clarity, and make sure you think about what length of notice period to include in your contracts of employment. As if you are dismissing or making employees redundant, then a long notice period can drastically impact your costs of such an activity.