Convert Time Variable to Numeric

Hello Everyone

I am new to R

I would like to know how to deal with Time Variable. If I have a column of
containing Time data in the format of hh:mm:ss such as 00:56:45, 01:24:36,
01:41:25, and so on......

I could find averages for this column using the Chron package. But now I
need to plot a graph so I need to convert this Time variable to numeric. 

My data is as follows:
Activity               Time Required
A                        00:56:45
B                        01:24:36
C                        01:41:25
D                        01:48:25
E                        01:51:25
F                        01:55:25
G                        01:59:25
and so on..............

total rows in my data are 150
Can someone guide me regarding converting this Time variable to numeric?

Thank you
RanjanV