That's also my experience with regular Ilford Multigrade developer. I replenish 1 liter "old" developer with 0.25 l freshly mixed developer at the start of every printing session. After about 2..3 hours, one liter goes back into a 1 L PET bottle (old mineral water bottle) and the rest down the drain. There's usually about a week between printing sessions for me. PET (recycle symbol "1") seems to be the most airtight plastic. I can't see any difference in the prints compared to when I make a complete fresh batch of developer.
I don't do this to save a bit of money. It's more about not wasting and minimizing the amount of chemical going down the drain.
Even when I kept the working strength LPD for months (there's a thread here where I posted about it, don't recall if it was two months or almost three, something like that) I could not tell any difference in d-max between prints made with the old stuff and a freshly mixed comparison batch. The contrast was perhaps .25 grade lower with the older batch, which made exact matching difficult (I didn't do split printing with half grade spaced filters to try to match that, and I doubt most non-photographers would have noticed
any difference in the prints) but other than the very slight contrast difference there was no discernible difference in the prints. If you want a really long lasting, commercially available developer, get Ethol LPD, with one caveat - the liquid concentrate is exactly twice the strength and roughly four times the cost per ounce, so twice as much per each measure of working solution as the powder to mix stock solution, so your dilutions change. Sometimes I consider the "not having to mix powders" worth the price, but if you do buy the concentrate, pour it into a different and squeezable bottle. The stock bottle is very hard plastic, you can't squeeze all the air out, and even what you do leaks once you break the foil seal. You can come back to find the bottle looking un-squeezed, and the life of the concentrate will reflect that. Pour it into a couple of squeezable plastic bottles you can seal and it'll keep as well or better than the stock mixed from powder.
Note this is about keeping the stock/concentrate, but the above trial I mention was indeed with mixed, previously used, working strength developer.