If you are doing a Ferric Reducing/Antioxidant Power asay (FRAP), that is one common protocol, as you will use the abs data to measure kinetics...so you take reading about every 15 s until completes 30 minutes.

http://pubs.acs.org/doi/abs/10.1021/jf9913458

Or you might use a fixed time limit about 10 minutes, for example.

http://www.sciencedirect.com/science/article/pii/S0003269796902924

There you have articles reporting both protocols and you can use the one most suited for your samples.

Ferric Reducing Antioxidant Power (FRAP) Assay

And the procedure was as following:

Various concentrations of sample and standard solutions (1ml each), 2.5 ml of phosphate buffer (pH 6.6) and 2.5 ml of 1% potassium ferricyanide were mixed separately and allowed to incubate at 50 o C for 30 min and 2.5 ml of 10% TCA was added to the mixtures and centrifuged for 10 min at 6000 rpm. About 2.5 ml of the supernatant was diluted with 2.5 ml water and is shaken with 0.5 ml of freshly prepared 0.1% ferric chloride. The absorbance was measured at 700 nm. 

You can take OD within 30 min after adding FeCl3.

Brief Protocol: Different concentrations/fix concentration of samples were prepared in sodium phosphate buffer (200 mM, pH6.6) and 2.5 mL of each sample was separately mixed with 2.5 mL of 1% (w/v) potassium ferricyanide. The mixture was incubated at 50 0C for 20 min. A mixture containing all the reaction reagents except the test material serves as the control. The reaction was stopped by adding 2.5 mL of 10% (w/v) TCA and the mixture was centrifuged at 1750 rpm for 10 min. The upper layer (2 mL) was mixed with 1 mL of 0.1% (w/v) of ferric chloride and made up to 8 mL with deionized water. The perl’s prussian blue color formed due to reduction in Fe3+was measured at A700. The EC50 of extracts were calculated from the graph of A700 versus extracts concentration.

Thanks all .. Your answers were so helpful for me