Those of you who have tried growing adult Lithops know that the first year or two are crucial. The plant you bought might have been either too small and too weak, or too big, overfed and stretched or it just might not have liked your environment and died due to acclimation. I've been growing Lithops for over 10 years now, and the good news is - once a plant has survived those first 2 years and found its most efficient size and shape, it will just keep going.
(You can find my detailed report on appropriate Lithops sizes and shapes - for windowsill conditions and all natural light - here, here and here.)
I have complained before that it was difficult for me to keep L. julii and L. karasmontana alive long. Looking back, it seems this was mostly because they tend to stretch. Getting a stretched Lithops back in shape is rarely successful. Getting a slightly stretched Lithops back in shape is possible but it takes a couple of years. And if it survives the process it'll be fine. In fact, once in shape it stays in shape.
I've had the below L. julii ssp. fulleri v. brunnea (C179) for 10 years now. This small round shape is, in my experience, the most "safe" one. Once my L. julii or L. karasmontana have it, they stay on course. Last year, it skipped regeneration, but regenerated just fine this year.
This is another example. I've had this L. julii since 2013 (bought in a hardware store). According to my photo records, it took the current shape around 2015 and kept it ever since.
The below two nameless L. julii are from the same hardware store bunch. However, after several stretched regenerations, they got into the round shape only this year. I'm hopeful they are now on the safe side.
This orange L. karasmontana shares the same story. I bought it slightly stretched in 2015 but for the last couple of years, once it settled down, it's been doing great.
This L. karasmontana ssp. karasmontana v. aiaisensis (C224) has been in my care since 2011. Going strong.
Of course it is best to make sure the plant is short and small from the start. Since most of us order plants online from a list these days, without knowing what they look like, it's not that easy.
Here are some plants I could select myself when I bought them back in 2015.
L. julii ssp. fulleri v. brunnea (C179)
L. karasmontana v. lericheana (C330)
Another L. karasmontana v. lericheana, 6 heads, bought last year. Short as the heads are, it should be fine now.
Hand-selecting is not always a guarantee though. I picked these two last year, and this year, one is very late with its leaf change. If it survives it will be late next year as well. I doubt anything good will come out of it.