Section 538 of the Thai Commercial and Civil Code mentions that a lease agreement MUST be registered if it's more than 3 years. You will found that the MAXIMUM lenght for a lease is 30 years under Thai (Section 540 CCCT). There are special regulations for commercial leases that can longer than that.

Please consult our text on lease in Thailand here: 

http://www.thailawonline.com/en/property/lease-agreement-in-thailand.html