Most of the spot ons will only last one month, and they do tend only to kill ticks, so you can find they still attach.
There are newer products available, I use Seresto collars, which work in a similar way to the spot ons, where the active ingredients absorb into the oils of the coat by direct contact, which means the collar must be fitted snugly but can last 8 months, and can reduce the numbers that attach dramatically.
Also, it is non toxic to cats, so much safer than the dog permethrin products that can kill cats when used inappropriately.
There are also tablets available, but these again don't stop the ticks until theyve attached.
With all the ticks and associated diseases, I feel much happier having something to repel ticks, as ticks, lyme disease and louping ill are rife around me.
I have never seen an effective 'natural' remedy.