Frequent, lengthy voice recording damaging to iPhone?
Not likely. Use your phone to do whatever you want to do with it.

Per iFixit's teardown there's a Toshiba MLC NAND flash chip in there. You've got at least 10,000 write cycles for the flash, which means you could fill the phone up 10,000 times before you had any problems. If you're figuring a gigabyte of recording each day, a 16GB phone gives you 40+ years of recording time.

(If somebody can find a datasheet for the part, you could confirm the potential number of write cycles. My 10,000 is an educated guess based on typical MLC.)