Time It Takes to Fly a Certain Distance -- Index -- V-Speeds Abbreviations List
If you don't have 70% of your takeoff speed by 50% of the runway, don't try it. This is a pretty good rule of thumb for gravel and grass runways.
example: If you don't have 50% of VR by 1500-1550 of a 3000 ft. strip, abort the takeoff.