This article is related with application of flowgraph model to estimate over the air specific RFID protocol execution time involving different number of RFID readers and RFID tag. It is possible to see application of large number of RFID readers to perform authentication process with RFID tag at the same time in RFID system. If there is an improper channel access algorithm implemented on RFID reader, it will cause readers-to-tag collision in RFID system. In order to resolve this issue, a proper timing of channel access on RFID tag should be estimated to avoid collision between RFID readers. In our proposed work, flowgraph model is recommended to be used as theoretical method for estimating over the air specific RFID protocol execution time required by different number of RFID readers and RFID tag in same zone. At the same time, we suggest that transmission channel access hold off time of each interfered RFID reader should take into consideration in order to avoid collision between RFID readers and tag. The transmission channel access hold off time of each interfered RFID reader is proportional with specific RFID protocol execution time for single authentication event involving a RFID reader and a RFID tag. With our proposed general function, it helps to provide reliability over the air specific RFID protocol execution time involving different number of RFID readers and tag in same zone.