Depends on the aperture. As aperture increases the CA increases and so a longer ratio is required.
If you Google Sidgwick or Conrady standard you'll find the answer, or...
There are two well known criteria for small achromats of standard glass BK7+F2 combination. Conrady states that the focal ratio has to be at least 5 times larger than aperture in inches. Sidgwick criterion is less stringent - according to this rule it is enough if the focal ratio is 3 times larger than aperture in inches.
The attached chart is useful (also here:
http://www.cityastronomy.com/CA-ratio-chart-achro.jpg)