Official text says animals keep earning Cash while you are offline, and the earners are the ones placed at your base. No rate is attached.
- Hatch something if the base is empty.
- Place it. An animal still in an egg, or hatched but not placed, is not the earner the description names.
- Expand if you want more earners. That raises the count, not a hidden offline multiplier.
- Leave. No time window is stated.
Spend the Cash the same way as online Cash: income upgrades, then more base space.
Unknowns: whether Cash is already on the balance when you rejoin or waits behind a claim; any cap; whether you must stay in a server. A missing cap is not proof of unlimited income.
Measure your save. Note Cash, stay away for a stretch you can count, and compare before you spend. One return a day later showed about 314,000 Cash at 4,000 Speed. That fits offline earning. It is not an hourly rate. Any precise Cash-per-hour quoted for this game is a guess.