Assuming you are asking how far one can see with a 20-60x60 spotting scope under ideal conditions, the answer is that one could theoretically see an object that is 3,280 feet away. This estimate is based on the fact that the average human eye can see an object that is about 1/60th of a degree in size. Given that there are 60 arcseconds in a degree and 3,600 arcseconds in a radian, we can calculate the distance to an object by using the following formula:
d = D * tan(a/3600)
where d is the distance to the object in feet, D is the distance to the object in miles, and a is the angular size of the object in arcseconds. Plugging in some numbers for our example (20-60x60 scope), we get:
d = 20 * tan(1/60)
d = 3,280 feet