Calculating battery time giving its capacity and load

No. First there are many many types of batteries, so arbitrarily defining Reserve Capacity, whatever that is really supposed to mean, as something related to a fixed amount of current like 25 A is absurd. A CR2032 coin cell, for example, can't ever provide remotely close to 25 A under any circumstances for any amount of time.

Your "reserve capacity" is in units of current times time (charge), which is also the units used for normal battery capacity. Normal battery capacity is usually expressed in A-h or mA-h, but this can be directly converted back and forth to your 25A-minutes.

You can as a rough first approximation estimate battery life time by dividing its capacity by the current you will draw. However, batteries are complicated, and this is far from a ideal relationship. If a battery is rated for 10 Ah, for example, then you will probably get close to 2 hours of use at 5 A and 10 hours at 1 A. But, currents both significantly smaller or larger can affect the overall capacity significantly. Large currents usually degrade capacity. At very small curents it can also be less because the self-discharge becomes significant. Temperature can also have a major influence on overall capacity.

The best way to estimate length of time a battery will be good for at a particular current is to examine the datasheet carefully. Look at the graphs of capacity versus current, what the cutoff voltage is used to determine "dead" and how that relates to your application, and the temperature this was all assumed at. The age of the battery will also matter, and the number of charge/discharge cycles if it's a rechargable type. Then realize these graphs generally show typical, not minimum guaranteed, values. Derate somewhat for worst case.

For particularly tricky cases, contact the manufacturer for guidance, and/or do a bunch of your own tests.

While you could use a "Energy density per volume" calculation, I think the safest bet would be to search for similar sized/priced/application batteries and make an estimate from there.

Since you know its 3.7V, I'll assume its Li-ion. From Wikipedia, we could say 250 to 620 W·h/L. Since your battery is roughly one milliliter, we can estimate from 0.25 to 0.62W.h, or 67mAh to 167mAh.

Now, we can double check that by looking for small Li-ion batteries on the web:

This one is a little bigger, offering 110mAh, while this other seems to be a bit smaller offering 40mAh.

From that I'd say the wikipedia numbers are a bit optimistic, because probably it takes into consideration the whole range of batteries (from the cheap ones we are looking at to others with super high density, etc..).

Next step is a guess, if you think this battery is new, good quality, from a good manufacturer, you can be optimistic. If its old, cheap and not taken much care for, then be a bit more pessimistic.

Anyway, its not a safe method at all, but should give a ball park. Ideally you would charge it and measure with different loads to be safe.