Answer:
I going to say its going to take her about 15 months to invest in the money to get the iPhone. But if you still don't get it, just take a look at my math.
Jenna wants to save => $35/per month
$35 * 14 mo = $490 + $20 = $510
(The $20 is for what Jenna started with, she most likely saved that up too)
So, it will take her about 15 months to get an iPhone.