The range runs from getting by on very little to living as comfortably as the environment allows, and where your friend lands on that spectrum depends on what you are able to contribute without affecting your own finances.
At Florida Women's Reception Center or any Florida DOC facility, inmates can spend up to about $300 per month through commissary. That is the high end and at that level your friend would be eating well by prison standards, keeping fully stocked on hygiene products, and covering phone costs and other incidentals without stress. That is genuinely comfortable for the environment.
Most people supporting an incarcerated friend or family member cannot or should not stretch to that level, and that is completely fine. Any consistent amount helps. Even $50 to $75 a month makes a meaningful difference in daily quality of life, covering basic commissary needs and keeping something on the phone account. The key word is consistent. A reliable smaller amount every month is more valuable than occasional large deposits because it gives your friend something to plan around.
The most important caution is to not let your support of her compromise your own financial stability. You have bills, obligations, and a life that needs to remain intact. Whatever amount you land on should be one you can sustain comfortably over the length of her sentence without strain.