In fiction, beings that can change shape can do so extremely rapidly and are immediately able to function in their new shape. While there is some precedent for the latter in nature (butterflies can fly more or less on emergence from their cocoon, and most ungulates can run within hours of birth), these critters are "hard-wired" for such feats.
On the other end, humans take months to years to learn to walk in childhood, or when recovering from major injury or very long periods of inactivity. (The scene from Kill Bill where manages to go from virtual immobility after months of being in a coma to driving a truck in the span of perhaps an hour is, at least in my experience, considered extremely unrealistic.)
Let's say we have a creature which has the ability to alter its form significantly. Body proportions and distribution of mass are considerably altered, perhaps even mass is significantly altered, such that the creature's balance and coordination is significantly different. How long, realistically, would it take the creature to learn to move again?
- Although a lot has changed, the creature's nervous system remains its own (unlike in this question); senses are mostly unaffected and nerves still control the same muscles. The creature is clumsy but not totally incapable of movement; it still has good control of what muscles move, they just tend to move too much or too little, and/or it needs to move in a way it isn't used to moving.
- The transformation takes about a month, but the creature is mostly sessile during this time. It has some very limited opportunity to move its limbs, but little or no opportunity to move around. (At least it doesn't need to re-learn how to breathe... that would end badly!) The transformation isn't necessarily permanent, but it's going to be keeping the new form for a while (months, at least; given it essentially "loses a month" every time it changes, it's not going to be doing this all the time!).
- The creature can't simply rewire itself to know how to move immediately. It doesn't have that ability, and didn't have that ability as an infant; it had to learn to walk the first time much like a human does. However, after the transformation, it does have the neurological malleability of a very young child.
- The creature was bipedal and wants to be bipedal again. The new form is reasonably suitable (about as much as, say, a two-year-old human) for bipedal locomotion.
- For the most part, the creature is moving around about as much as an average human toddler (it is a toddler after all, at least in the literal sense!) that will naturally help it improve, but isn't specifically focused on doing so. However, it is also engaging in 1-2 hours of daily targeted exercises designed to improve balance and coordination.
Now, it probably goes without saying that this will be a long process and "success" is fuzzy. So for the sake of being able to give a reasonable answer, let's say that I am specifically interested in how long until the creature can walk (bipedally!) with only occasional falls. Let's also say I'm specifically interested in the first time it tries to master a(ny) new form; as Willk rightly observes, the situation might improve dramatically with practice.
(Note: I've tried looking for information on humans learning to walk, but while it's trivial to find information on when we start walking, it's much harder to find information on milestones once they start. The only source I managed to find was a youtube video that gives six months from first steps to running. Is this reasonably accurate? Is this even a reasonable model for the situation of my creature? Would it be plausible for my creature to get to the "usually doesn't fall over" stage quicker; say, in a month? Running can take longer; I just want walking without falling over.)