Condition a new capillary column at approximately twenty degrees higher than the final temperature of your oven program without exceeding the upper temperature limit of the column. If a temperature higher than the isothermal temperature limit of the column is needed for your analysis, recondition the column at that higher temperature, but, again, don't exceed the upper program limit.
When you install your column, purge it with at least three volumes of carrier gas prior to ramping it to the conditioning temperature. The total column conditioning time will depend on the type of application you're running and how much bleed is acceptable. The lower the detection limit that's needed, the longer the column will need to be conditioned. (Column bleed is closely related to the polarity and the film thickness of the stationary phase.) Polar and thick film columns bleed more and require more conditioning. For most applications, 30-60 minutes of conditioning is usually sufficient.
But how can you really determine when a column is sufficiently conditioned?
A flame ionization detector (FID) works best for monitoring the baseline during conditioning. Toward the end of the temperature ramp (i.e., 30-40?C below the isothermal upper temperature limit), the baseline will rise, then come down and level off, at which time you may consider the column conditioned. There are those that report detector fouling during conditioning when using other types of detectors (e.g., ECD, MS), but it's generally considered a safe practice to condition the column while connected to these detectors.
One more thing: don't condition a column overnight. Column life expectancy is greatly reduced when the column is stored at high temperatures. If you're experiencing an excessive amount of bleed for more than two hours, bring the oven down to room temperature and locate the source of the problem (usually oxygen entering the column from loose fittings or a leaky septum). Baseline signals that mimic column bleed can also originate from residues present in the GC itself.
One more note: if the column has not been in use for a while, a mild conditioning step may be needed to drive off contamination which may have condensed inside the column during storage. Also, there is nothing to suggest a limit to the ramp rate of the oven when conditioning a column