None of the references I could find go any longer than 150ft; however, here are a couple of points:
- Extension cords are not designed for permanent use
- Extension cords should not be run inside of conduits as they rely on the surrounding air for cooling; running in a conduit would derate the ampacity (no charts for how much because you're not supposed to do it). Someone with a copy of the Electrical Code could give you an estimate (my NEC handbook was from the 80's so I got rid of it a long time ago).
- 150ft Extra Heavy Duty #10 extension cord is only rated to a maximum of 15A, which may be what one of your heaters draws
- 250ft #10 may support 7A, but with a significant voltage drop??
Please get rid of the extension cord and have an electrician run an adequate service from your main panel down to the shed before you have a fire.