The initial BOINC estimates are way off, about 4 hours that I recall (I run the 24-hour work units). So until they correct themselves, I always get too many when first attaching.

To speed up the correction, you need a cc_config.xml file placed in the BOINC data folder, with this entry:

<cc_config>
  <options>	
  	<rec_half_life_days>1.000000</rec_half_life_days>
  </options>
</cc_config>

You then need to activate it by reading it in, or else restarting BOINC.
It should correct the time estimates in a couple of days.

cc_config options require a client restart as it is only read at client startup. app_config can be read.