How about 'period'?

using a verb instead of a noum "lasts 4 times "

I believe that it is the chromatogram that should be recorded for four times the retention time of the major peak

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Note added at 46 mins (2005-08-31 19:39:30 GMT)
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My conclusion is based in particular on the following:

Gas chromatography is the physical separation of two or more compounds based on their differential distribution between two phases, the mobile phase and stationary phase. The mobile phase is a carrier gas that moves a vaporized sample through a column coated with a stationary phase where separation takes place. When a separated sample component elutes from the column, A detector converts the column eluent to an electrical signal that is measured and recorded. The signal is recorded as a peak in the chromatogram plot.
Chromatograph peaks can be identified from their corresponding retention times. The retention time is measured from the time of sample injection to the time of the peak maximum, and is unaffected by the presence of other sample components. The height of the peak relates to the concentration of a component in the sample mixture.
http://www.mtholyoke.edu/courses/sdecatur/GCMS/intro.html

A duration is how long something happens or takes, and you wouldn't normally say that you record a duration. Also, 'according to xxx' in your construction means 'xxx says/specifies...', which isn't what you mean.

I'd suggest 'Using the above-mentioned method [or: the method described above], the sample was injected and the results were recorded for a duration of four times the interval until the first peak.'

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Note added at 47 mins (2005-08-31 19:40:49 GMT)
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Sorry, that should be 'four times the retention time of the major peak'