Leaving them fully charged for 4 to 6 weeks is not a big deal. If your serious about getting your monies worth, then you might want to consider a smart charger instead of the iMac charger, My batteries are over 3 years old and they keep coming back to life, have yet to through one away. I own about 20 of them or so.
A smart charger like the TITANIUM SMART FAST 8 BAY AA/AAA BATTERY CHARGER, form Battery Junction, charges each battery independently in each bay. Plus after you have charged your batteries 15 or twenty times, the smart charger has a built in discharge button. The discharge brings the batter to an almost zero state and then the charger automatically charger the batter back up to the full 1.5 volts.
I purchased an 8 Bay smart charger for $27.95, but I think you can buy the same charger with a 4 bay cheaper. Big plus, the charger (my 8 bay anyway) come with the DC charger so I can take it with me on the road and keep my batteries fully charged. I use the same batteries on my iMac things, MM or key board, plus I have flashlights that use this same chemistry, so it all works well for me.
You can charge a NIMH's and leave for 3 months unused with no problem at all. After about a year or so, it will start to drop, but should be good to go with a least an 80% charge. I try to catch my batteries right around the 40% discharge although I have had some go down to the 20% mark and it did not hurt them. I charge each battery around 15 to 20 times then I drain each battery using the discharge button and then go back to fully charging them.