At 8 MHz,
processor has a clock period of TCycle = 1/8 M = 0.125 μs
Instruction cycle = 10 × 0.125 = 1.25 μs.
To check status requires one input-type instruction to read the device status register.
At least one other instruction to examine the register contents.
If the device is ready, one output-type instruction is needed to present data to the device handler.
The total is 3 instructions requiring time T = 3 * 1.25 = 3.75