Note that increasing the respawn time to really high values can lead to issues later on as the game has to keep track on more and more stuff. So you'll get larger save files etc. I remember dev_akm warning about it, but those posts are of course pruned long ago.
I think the consensus (including my view) now, is that this is mostly a myth.
With a long respawn time, cells will never be reset, so the game have data for all killed monsters, etc. in your savegame forever. But with a shorter respawn time, a cell still doesn't get reset until you actually load it (by entering it, or an adjacent outdoor cell). The moment the cell gets loaded (and the respawn timer is passed), all dead monsters, etc. gets removed (thereby also from your savegame), but at the same time new monster spawn, so the net number of records stored in your savegame is about the same.