Hi mate and welcome aboard. You will initially sign up for four years.
I am not 100% on how binding this is actually is and there are always ways out, however I guess most people leave well before they get the lid, do continuation training and go onto to do an SQ. If you make it that far which could take over two years why you would then want to get out would astound me.
I just found this on the RN site:
'Premature Termination of RMR Service
Unless you are in permanent service or Full Time Reserve Service under Section 24 of the Reserve Forces Act 1996, or an order is in force authorising the call out of the reserves in the circumstances described above, you are entitled to ask to be discharged before the end of the period for which you enlisted or re-engaged. You will also be required to give one month's notice in writing and return clothing and other public property issued to you in good order or pay for the deficiencies.'
That is the long way of saying four years with entiltment to leave with permission.
Where is your local detachment?
ATB
Wulf