It takes from 1 to 2 weeks to collect all the necessary documents and complete an application form correctly. If you book a consultation with qualified immigration advisers, the preparation will be reduced to a minimum.
It would normally take 3 weeks for your application to be processed, provided you have completed the application form correctly, confirmed your identity, submitted all the necessary documents and paid a relevant fee. Once your application is approved, you will receive further instructions either by post or by e-mail.
To sum it up, if you are supported by qualified legal experts, it will take about 4-5 weeks to get a UK Marriage Visitor visa.