Fast wireless charging is only 1.4 times faster than regular wireless charging. The phone may get warm during the process, but probably is never any more hot than when playing a game for the same length of time. Furthermore, the charging process slows to a trickle when the phone is completely charged. If the phone remains turned on while being charged, I would imagine that the trickle charge is being consumed by the phone's system and idle time power consumption. So I wouldn't worry about leaving the phone on the wireless charger all day while at work. It shouldn't measurably hurt it.
Even Samsung claims that after 2 years usage, the N8's battery will maintain 95 percent of its original capacity.
The worse case scenario would be you get the battery replaced sometime down the road. Any repair place should be able to do that the same day.
Having said all of the above, what will more likely shorten your batt life prematurely is depleting all of its power before recharging it. Personally, I don't like to let my phone's tank get below 25 percent.