If you plan on actually observing one keep these in mind:
#1 Actually cutting off a arm yourself can often cause a fungal infection, hence excellent water quality would be required.
#2 Certain starfish have little to no regenerative capabilities and will regenerate arms VERY slow, Brittle stars are among the quickest and fastest in terms of regeneration (they break off thier arms to escape preditors, hence the name "Brittle") and can usually start to see recovery within a week assuming its in good health. Shrilanka's are an example of poor regeneration, and can often take weeks before any signs of recovery are noticed, again assuming the health of the star in question.
If you dont plan on actually preforming an experiement on a live one, its often hard to find information on just arm recovery, since most studies are done with actual aesexual cloning, as in actually cutting a starfish in half and both halves regenerating into 2 complete organisms.