Where x = delay time in milliseconds:
BPM =  60,000/x
For example, for a delay of 250ms:
BPM = 60,000/250 = 240 BPM
To find delay in ms when given the BPM, where y = BPM:
Delay = 60,000/y
Thus, if you want an echo on the beat at 120 BPM
delay = 60,000/120 = 500ms
This is because there are 1,000 miliseconds in a second and 60 seconds in a minute.