Estimating CAN Bus Load
To estimate the bus load you need two pieces of information for each msg on the bus


Total message bits ( data bits + protocol) 
Message rate


You also need the Bus Rate.

You first need to determine the total bits consumed (bps) for each message. This is message total bit x message rate. The summation provides the total bits in bps.  This can then be compared with the bus rate.

say you had three messages
ID1 = 55bits every 1ms   = 55kbps
ID2 = 87bits every 10ms  = 8.7kbps
ID3 = 111bits every 1s   = 0.11kbps

total kbps is therefore 63.81kbps.

thus the bus load is


25.5%  at 250kbps
12.8% at 500kbps
6.4% at 1Mbps


so if you have one message that is 128bits being sent at a rate of 1ms, that is 128kbps. On a bus that supports 250kbps, this is a bus load of 51%