So to the original question, to me knowing when to refill a woodstove is just based on a feeling you get with experience using it, no need for any fancy gear. As you use and learn the system, you get a sense of timing in terms of how long a certain load of wood will last, in addition to visuals like a glass door obviously, or simply feeling the current heat output as you walk through the room it's in. And at the end of the day, as long as the box is still warm and you've got some coals to kick it back up again, that's all you really need to aim for if you need a constant heat source.
As long as this has turned into a general firewood heat discussion, guess I'll share mine as well, and honestly I pretty much have it down to a science.... The house is a funky 60s thing, stoves are 90s era Avalon I got free from a customer, and I don't own the house so it's not exactly what I'd do given the choice, but what I have works great. To move heat from the stoves into the room and around the house, I have fans blowing across them. My record is the fire not going out for 3 months straight, and I live alone so no one else to keep it fed when I'm away 12 hours at a time.
Kindling is old cedar fencing from a customer. Not used often, because once I start burning in the fall I keep it going until spring basically, unless I go away from the house overnight or accidently fall asleep on the couch watching a movie or something. I start it with some non-flareup woodstove juice and don't bother with that newspaper nonsense.... On that goes the small split doug fir which burns longer and hotter than the cedar obviously, and this is the next step to getting the box hot. I also use this to kick up coals fast before adding the bigger wood if needed. Then the actual firewood is all doug fir, sourced within a few hundred feet of the house. Down for ~2 years, split in size ratios of how much medium wood I want when I'm home and awake, and larger pieces for when I'm away at work or sleeping and need longer burn times. 100% bark free.
I do get a lot of creosote buildup because I mostly run the primary stove dampered down all the way, and the houses design of the metal pipe that goes straight into a large masonry chimney causes rapid smoke cooling, which leads to buildup on the insides of the masonry. I sweep it once a year, and it's not a big deal. Generally I only run the one stove which is in the kitchen, and the second stove in the living room when it gets below freezing, which isn't often here. Next to that stove though is a large in-house wood storage area, and that gives me about a week of wood before I have to go outside to get more
