The start and stop times of an observation are recorded in the TSTART and TSTOP keywords. The values are in seconds since the MJDREF keyword; the Chandra reference epoch for flight data is 1998 Jan 1, 00:00:00 TT, or MJD 50814.0 TT.
The "length" of an ACIS observation is stored in the LIVETIME keywords. This set of keywords gives the exposure time, in seconds, for each of the ACIS ccds. The LIVETIME keyword gives the exposure time for the chip that contains the aimpoint of the observation. Note that this information is also stored in the EXPOSURn and EXPOSURE keywords.
The ONTIMEn keywords give the amount of time that each CCDs is considered to be taking "good" data (i.e. the sum of all the GTI periods for a chip). These values are generally larger than the corresponding LIVTIMEn values because they include those periods when a CCD is on but not taking data (e.g. when it is reading out the data to the frame buffer).
For interleaved mode data there will be a set of these keywords (other than TSTART and TSTOP) for each mode.