Must I abide by the oath my mother forced upon me, or has my oath been dissolved, and do I owe a separate expiation for each day I used the phone for more than an hour?

The oath is broken by the first instance of violating it. The violation does not repeat with the repetition of the act, unless the swearer intended repetition, or if the wording of the oath implies it, such as "every time." Accordingly, the questioner's oath was broken by the first violation, and she is not required to offer expiation if she returns to doing what she swore to abandon, due to the absence of an intention for repetition or wording that implies it.