Hey can i just ask how long before my visa expires do i have to apply by post?

For example, can i apply for set(lr) through post on 25th of September if my visa expires on the 29th? Or is there a certain time before my visa expires that i have to apply?

Hello Everyone, i am hoping i can apply for the SET(LR). However, im not sure about a few things of which i am hoping someone could help me out. I first came to the UK 01/04/2005 on a dependent tier 4 visa as my parents were studying and switched to an independent tier 4 visa on the 24/09/2013. My c...