Shulchan Aruch OC 238:
א. צריך ליזהר בלמוד הלילה יותר מבשל יום, והמבטלו עונשו מרובה
ב. אם יש לו חק קבוע ללמוד כך וכך ליום והיה טרוד ביום ולא השלימו, ישלימנו בלילה מיד
Regarding upper limits, I believe this is a Nedarim question. The Gemara says (Nedarim 7a):
אמר רב גידל אמר רב: האומר לחבירו: "נשכים ונשנה פרק זה", עליו להשכים
Shach (YD 203:5) writes that even if one just says "I will learn x" without using the language of a neder, he is bound by these words. Moreover, if someone decides to accustom himself to doing a certain good thing and his plan is to always continue doing so, it retains the status of a neder even after one time (Shulchan Aruch YD 214:1). According to the Kitzur Shulchan Aruch (67:7) if one does it three times then it retains the status of a neder even if his plan was not specifically to always continue doing so.
All this being said, it would seem that there is no problem with specifically planning to always learn at least a mishna, or something like that. Since you have a specific plan, I would say that doing extra every now and then doesn't mean anything nedarim-wise. However, if every day you do an "extra" mishna, and you don't continually specify that you are still sticking to your original plan and this is just an exception, it could be that your "plan" doesn't help. I'm guessing here.