yes i cheated by using prems, in fact it made it too easy. Only got 2 hearts with basic ammo. How long you survive basically depends on how much energy you have to boost.
0:00 - Heart 1
6:00 - Heart 2
11:30 - Heart 3
16:55 - Heart 4
22:00 - Heart 5
26:55 - Heart 6
32:00 - The Final Blows