It depends, my sis-in-law is off for 14 months because she has saved holidays up. I think it also depends on your contract. Her contract allows them 11 months maternity leave GP's, I believe, are off for 6 months.
It doesn't 'depend' on your contract or what the company offers (unless you are incredibly lucky to get extras on top of legal entitlements) or anything else.
Official and legal entitlement to Mat leave is 12 months with 9 months paid at the 'going rate' - not sure what that is now but Box has posted the link to it all
You may be able to save holidays but that's not usual. You should take them at the beginning of your ML so as not to lose them and you will need to ask your employer if you can do this. There is no legal requirement for your employer to allow you to save them or carry them over from one year to the next