Hi we applied and got 6 month visitor visa for parents valid till mid may of this year, they have traveled for the time period they stated, but now they will need to visit after mid may so when can they make the second application for visitor visa for another 6 month visitor visa ?
Applications for a UK Standard Visitor visa can only be made up to 3 months before the expected travel date https://www.gov.uk/standard-visitor-visa so if your parents want to visit from mid May they’ll have to wait until mid February to apply. Depending on their circumstances, if they have a good reason to visit regularly and a strong travel history/ties to home they could consider applying for a long term Visit visa.
A word of warning about back-to-back visas: it’s not against the rules but your parents should be careful not to give the impression of trying to live in the UK through frequent, extended or successive visits. See Immigration Rules V4.2(b) https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rules The typical rule of thumb is to spend at least the same amount of time outside the UK as in it.