Resigning from job?

Once you have completed any probation period notice will be 6 weeks. Once you have been in post for 5 years it can be longer.
The period of notice always starts from the 1st of the month after it is received by the employer.
Two things to consider:
A new employer will know that a new employee will have to give 6 weeks notice to their previous employer.
There is nothing to prevent a negotiation to allow an employee to leave before the 6 weeks is up. As an employer I never wanted to keep an employee who wanted to leave - they are unlikely to work to their full potential when all they want is to move on.