How long for new lawn?

If I wanted to kill the grass by covering up and then seeded a new lawn after the old one died off how long would this take in total ?

I relaid my lawn 3 years ago from turf. I reseeded parts that were dying off but used hardy seed rather than luxury seed which must have been what the turf was grown from so now I have a mix of luxury and Hardy grass plus some moss and weeds I'm wondering if the best way to sort this out is to start again I'm open to any other suggestions.

 

This would be better posted in the 'Lawns' forum men8ifrfor.