How likely is it that your nexplanon will migrate in your arm? and how long does it take to encapsulate in your arm?

It happens: Nexplanon like its' Norplant predecessor can migrate in your arm especially if there are body weight changes with fat added to your arms. Encapsulation occurs when the body sends out a reaction to the implant and surrounds it with "scar" tissue making extraction more difficult. The length of time for this to occur is dependent on the individual patient's body reactions.

Unlikely;days/weeks: The possibility that Nexplanon "will migrate in your arm" is fairly small, probably 1% or less, particularly if it was inserted properly and in your non-dominant arm. The rate at which your body trigger an inflammatory reaction to "encapsulate it" will probably vary between days to weeks depending on your health status, immune-wound healing response and other metabolic factors.