AST 6 months or 12 months tenancy agreement?

[Rayking]

First I would like to thank everyone that has contributed to all my questions in the past 6 months.I've patiently been asking questions in the past 6 months to avoid spending twice for fiancee visa and a lot of people on here have been of help.my fiancee will be applying in a couple of weeks and hopefully everything goes as planned.
My question is about Assured shorthold tenancy,I've got my landlord to sign a properly written one as he only gave me one page agreement before.
I have an option of putting 6 months or 12 on the agreement, my question is,is AST 6 months or it can be 12 months agreement? If six months,is it Ok if my landlord just state I can have the room as long as I want to cover,because the tenancy will be less than 6 months by the time we apply? Or I can just use 12 months.
Pls,your expertise will be appreciated.
Thank you all and may you all be rewarded for your good works and help.

[SoHopeful]

I don't think it matters, however if you are able to get a 12 month agreement that would be the best option.

[Rayking]

Thanks you, also for your responses in time past, you've really been of help.

[SoHopeful]

Your welcome