It's up to you.
You asked the question - after clutch starts to slip, how many miles have I got?
This is a question similar to: How long is a piece of string?
The answer is, it is determined entirely by the rider and his clumsiness/ cack handedness.
Thus, My brother had a car with a slipping clutch. He drove sympathetically, using the lower gears more, & backing off the throttle a little if it slipped. He drove it for two years like this.
I have done similar in a car for six months.
On my last 1100, clutch slipped at speed for a year before I changed it. this included a trip of over 4,000 miles round Europe in 8 days.
A cack handed person will keep the accelerator open when the clutch slips, and maybe get another kilometre or so before clutch expires completely.
So, the answer is: It's up to you!
Myke