I don't know which radio you purchased...

Most TX's come with fairly low capacity battery packs. I typically toss these and purchase something with higher capacity.
As such I don't see any loss to the removal of the battery packs, especially if it saves me a few dollars on the TX.

As far as charge time, take the capacity of the battery and divide it by the output amperage of the charger.
Nothing to it.

So if you are using a 50mA charger it will take 54 hours to charge your pack from a completely discharged state.

Note however that you may not be able to charge a pack to the full rated capacity, nor discharge it completely so actual charge times can be less than this. Also inefficiencies come into play as well.

There are plenty of "charge time calculators" on-line ( hint: google ).

Re: Power

By the time your packs hit 3.5v it is pretty much discharged completely.
The electronics will not receive sufficient operating current when the packs are down to this level.

Most servos can use up to 1 to 2 AMPS(!!!) each when fully stalled... so those 650mAh batteries are practically a joke.

In actual operation we tend to use less than that.

I saw an article for beginners today that recommended this battery pack formula as a STARTING point...

Battery Pack capacity needed on plane = 1000mAh + ( 250mA * Number of Servos used in Plane ).

That is more realistic.