The number of chips granted for a rebuy/addon varies somewhat from one site to another, and one must adjust accordingly. Subject to those considerations, the standard strategy is simple enough:-
Budget adequately for a rebuy tournament. You must assume that the cost will be at least 3x the buyin (ideally: Start, 1x rebuy, 1x addon). If this seems too expensive, do not enter. Some sites (example – Ultimate) allow a double rebuy and a double addon in some rebuy tournaments, in which case your investment will be 5x the buyin (although a better plan here [since the addon is larger] might be one rebuy and two addons for 4x in total). Ensure that you know the structure beforehand.
You should rebuy as soon as the rules permit and then seek to double up. The objective is to have reached at least 4x the starting stack by the end of the rebuy period, ie. starting chips + 1 rebuy x 2.
During the addon period, it may be better to save the money if you are not doing well and the extra chips will not put you in the upper half. At the other extreme, it is arguably unnecessary to spend more if you are doing well and are already among the top 10 percent. In all other circumstances, take the addon.