You have a 6 month window starting on 1st December to buy the SAYE shares at the option price of £3.60. When you then sell them is up to you.
Or you could just take the money out of your savings account at any time if you choose not to buy your shares. Full details are in the SAYE booklet, available here: https://www.myroyalmail.com/saye
The allocation dates of the free RM are as follows:
15th October 2013
9th April 2014
5th October 2015
6th October 2016
You can sell those after 3 years of each allocation date if you’re prepared to pay tax & NI, or 5 years if you’re not. If you leave RM as a ‘good leaver’, you won’t have any tax to pay. Details here: https://www.myroyalmail.com/employee-share-offers