Help! Calculating Wait time

Try this and see 

Add this column to your appointment table!

Column Wait Times = 

-- Get the invoice id of the same patient id and same date
var _inv_pid_appt2 = CALCULATE( MAX(Invoice[_id]), FILTER( ALLSELECTED(Invoice), Invoice[patient_id] =  appointment[patient_id] && Invoice[date].[Date] = appointment[Start_time].[Date]) )

-- Get the checked in record time of the same invoice id
var _inv_statushis_pid_appt2_checkedIn = CALCULATE( MAX(Invoice_statushis[Created_on_Time]), FILTER( ALL(Invoice_statushis), Invoice_statushis[invoice_id] = _inv_pid_appt2 && Invoice_statushis[Status] = "Checked In") )

-- Get the consult start record time of the same invoice id
var _inv_statushis_pid_appt2_consultin =  CALCULATE( MAX(Invoice_statushis[Created_on_Time]), FILTER( ALL(Invoice_statushis), Invoice_statushis[invoice_id] = _inv_pid_appt2 && Invoice_statushis[Status] = "Consult Start") )

RETURN 
IF (
    _inv_statushis_pid_appt2_checkedIn < appointment[Start_time],
    DATEDIFF(_inv_statushis_pid_appt2_consultin, appointment[Start_time],  MINUTE),
    DATEDIFF( _inv_statushis_pid_appt2_consultin, _inv_statushis_pid_appt2_checkedIn, MINUTE)
)

First, I tried getting the invoice id for the same patient id and patient appointment date.

Second, I used the invoice id and then filter the checked in and consult start events and get the max time 

Third and last step, tried to do your logic calculation.  You may have to tune further this step as it does not make sense purely if statement vs true condition calc. My thinking says as