How to increase data transmission speed between cc2500 and cc2511?
8 million bits in ten minutes is 13.33k bits per second. Shouldn't be a problem if your max throughput is 250k bits per second.

Max time depends on how far the tx and rx are from each other. More than a certain distance nothing will be received. Close together 8Mb will be sent in 32 seconds at  250k bps.