I've learned a rule of thumb by a Cisco CCIE and he said, that if a link is utilized more than 50% on average over a period of a workday, you should consider upgrading the connection.
Here's an official way of calculating Link Utilization and Efficiency:
Link utilization is simply the average traffic over a particular link expressed
as a percentage of the total link capacity. Link efficiency is a less
commonly used term that is defined as the ratio of the time taken to
transmit a frame (or frames) of data to the total time it takes to transmit
and acknowledge the frame or frames:
Utilization (U) or Efficiency=(Time taken to transmit frame)/(Total transmission time)
The efficiency of a link with stop-and-wait ARQ can be determined as follows:
If the time taken to transmit a frame or block of data is tf, the propagation delay for both frame and acknowledgement is td, the time taken to transmit an acknowledgement is ta and the total processing time is tp, then:
U = (tf) / (tf + ta + tp + 2td)
In many situations the acknowledgement transmission time and processing times
can be ignored, giving:
U = (tf) / (tf + 2td) = (1) / (1 + 2a) ; where a = td / tf
A point-to-point satellite transmission link connecting two computers uses a stop-and waits ARQ strategy and has the following characteristics:
Data transmission rate = 64 kbps
Frame size, n = 2048 bytes
Information bytes per frame, k = 2043 bytes
Propagation delay, td = 180 ms
Acknowledgement size, ta = 10 bytes
Round-trip processing delay, tp = 50 ms
Determine the throughput and link efficiency.
Frame transmission time tf = (2048 x 8) / (64000) = 0.256 sec.
Acknowledgement transmission time ta = (10 x 8) / (64000) = 1.25 msec.
Total time to transmit frame and receive an acknowledgement is:
tf + ta + tp + 2td = 0.256 + 0.00125 + 0.05 + 0.36 = 0.66725 sec.
Throughput k = (2043 x 8) / (0.66725) = 24.494 Kbps.
Note that the resulting throughput is considerably less than the transmission rate of 64 kbps.
The link efficiency (Utilization) can now be calculated, neglecting ta and tp, as follows:
a = (td) / (tf) = (0.18) / (0.256) = 0.7
U = (1) / (1 + 2a) = (1) / (1 + 1.4) = 41.67 %.