He has to move troops in around Feb 15 and has to get them out by the end of March?
This is due to weather conditions on the ground.
The ground freezes Feb 15, at that time he can safely deploy ground troops, trucks, tanks. At the end of March the spring thaw makes the ground too wet and soft. I would think he has to get most of the troops in and out by then or they will be stuck for awhile?
Putin may not need more ground forces than he has now.
Air power and missiles can be used in addition to land battalions.
Air power and missiles can be deployed rapidly.
In past wars, air support has been used to soften the target so the ground troops have fewer casualties and can make faster progress.
Putin would send in air power and missiles before Feb 15. He probably doesn't need a lot of time to shock and awe the Ukraine, their defense is poor. Then he sends in the land troops on Feb 15. Then get them out by the end of March with a smaller occupying force left behind.
Ok I am not a general and don't play one on TV. I am a student of WWII battle plans. This is my best guess war gaming this out.