Battery pack is fully charged.

Connected phone, charged for 10 to 97%.

Now discharging phone phone for second charge cycle. (BT on, wifi on, gps on, installing apps, streaming youtube and recording video, screen on highest brightness)

Charges from 1 battery-pack:
10-97%
5-99%
4-99%
10-99%
10-20%
low voltage light goes on, charging is terminated.

So, 10Ah battery can charge the phones 2.1Ah battery with 375%, making this whole conversion cycle rated at ~80%, which is not bad.

If the assumption is correct that doubling the battery to tablet size and doubling the battery pack to 20Ah gives the same results, you could easily use it for a week with normal use.