Earliest date for renewal of visit visa

[ilruk84]

Dear All,

My parents have visit visa valid until May 2018 (issued on May 2013 for 5 years) from Pakistan, they are planning to visit us in June/July (my mother is school teacher hence she ideally want to come in summer holidays). My query is when is the earliest they can apply for renewal for visit visa? We are planning to apply in April 2018 will this cause any problem?

Many Thanks

[ilruk84]

Anyone have any information on this or have past experience about it???