It depends on what train are you going to take. Wuhan Railway Station only has the G trains to Guangzhou North or South Railway Station. Which only takes some 3 hours but costs 490/780RMB for second/first class seats. There are other Z, T or K trains which takes 10-15 hours. They are cheaper but available from Wuchang Railway Station and mainly to Guangzhou Railway Station. So, you can choose according to your budget, time and destination (closer railway station).