You can actually see a snapshot of the utilization by doing "show interface xyz" for your interface between the routers. If RouterA used Serial0/0 to connect to Serial0/1 on RouterB, you could do: RouterA# show interface serial0/0
You would see something like:
RouterA#show interface serial1/0
Serial0/0 is up, line protocol is up
Hardware is DSXPNM Serial
Internet address is x.x.x.x/30
MTU 4470 bytes, BW 44210 Kbit/sec, DLY 200 usec,
reliability 255/255, txload 107/255, rxload 44/255
Encapsulation HDLC, crc 16, loopback not set
Keepalive set (10 sec)
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/144418/0 (size/max/drops/flushes); Total output drops: 28238450
Queueing strategy: fifo
Output queue: 0/40 (size/max)
30 second input rate 7784000 bits/sec, 1917 packets/sec
30 second output rate 18710000 bits/sec, 2720 packets/sec
3985575203 packets input, 4114474167 bytes, 962 no buffer
Received 1429313 broadcasts, 0 runts, 0 giants, 0 throttles
3273 input errors, 3273 CRC, 623 frame, 253 overrun, 0 ignored, 472 abort
1632477877 packets output, 1915617409 bytes, 0 underruns
0 output errors, 0 collisions, 34 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out
1 carrier transitions
DSU mode 0, bandwidth 44210, real bandwidth 44210, scramble 0
The bolded lines give you a snapshot of the bits/sec. Here my inbound is 7784000/1024 = 7601kbits/s or 7.601megabit/s. Outbound is 18710000 / 1024 = 18271 kbits/s, or 18.271 megabits/s. The best thing to do to keep historical data is to use Cacti or MRTG or PRTG or any other SNMP polling device to capture this data.
