These police officers were suspended for 4 years on full pay.
http://www.sundayherald.com/news/heraldnews/di splay.var.2511504.0.police_under_fire_as_two_s uspended_officers_spend_four_years_on_full_pay .php
This doctor was suspended for 12 years.
http://www.independent.co.uk/news/doctor-was-s uspended-on-full-pay-for-12-years-1596698.html
There is no set maximum period of suspension but the employer is expected to come to a decision as soon as is practicable. If the suspension is because of pending criminal charges, it would be reasonable to wait for the verdict. If it is because of suspected breach of discipline or policy, a decision should be made when all the investigations are over.