Sonar DataSheet here:

Timing Description

250mS after power-up, the LV-MaxSonar®-EZ4™ is ready to accept the RX command. If the RX pin is left open or
held high, the sensor will first run a calibration cycle (49mS), and then it will take a range reading (49mS). Therefore, the
first reading will take ~100mS. Subsequent readings will take 49mS. The LV-MaxSonar®-EZ4™ checks the RX pin at the
end of every cycle. Range data can be acquired once every 49mS.

Each 49mS period starts by the RX being high or open, after which the LV-MaxSonar®-EZ4™ sends thirteen 42KHz
waves, after which the pulse width pin (PW) is set high. When a target is detected the PW pin is pulled low. The PW pin is
high for up to 37.5mS if no target is detected. The remainder of the 49mS time (less 4.7mS) is spent adjusting the analog
voltage to the correct level. When a long distance is measured immediately after a short distance reading, the analog
voltage may not reach the exact level within one read cycle. During the last 4.7mS, the serial data is sent. The LVMaxSonar
®-EZ4™ timing is factory calibrated to one percent at five volts, and in use is better than two percent. In
addition, operation at 3.3V typically causes the objects range, to be reported, one to two percent further than actual.