As the law currently stands, a woman who has given birth is entitled to take up to 52 weeks of maternity leave. This right applies regardless of the employee's length of service. 

Once the leave has been used up (or if the woman decides to cut the leave short), the woman is usually entitled to return to the same job as that which she left. 

In addition, the woman is entitled to take time off work whilst pregnant to attend antenatal appointments and may qualify for up to 39 weeks of statutory maternity pay (which is currently paid at the rate of £139.58 per week). 

I have seen many employers be more generous with the pay entitlement (perhaps paying up to 6 months at the employee's full rate of pay) but, in my experience, I have not seen any employer grant a longer period of time off work. It will be interesting to see whether the comments referred to below generate any appetite to give more than a year off work.