Could anyone kindly tell me, how long it would take to make a thorough tour
of this garden? I am still planning the "English gardens visit", this
garden lies between the airport and our staying place, but we cannot afford
to spend too much time walking there - will two hours be sufficient?

Regards, Barbara.

Since their website says visits strictly by appointment, you will have to contact them anyhow. Why not ask their advice? The formal Gardens will not take too long, but the Wild Garden and other bits may take a lot longer.