There is no universal number, and anyone quoting one is selling something. You set the lifespan yourself, by what the account does every day.
What it depends on
Load: how many messages to strangers, how many chat joins, how many actions per hour. Evenness: a human rhythm against a script's. The quality of the account itself: age, idle time, history. And the proxy: a permanent private address against a shared one.
The first factor dominates. An account running fifty per cent above the norm does not live a third less — it lives several times less.
How to measure your own
Record the date each account entered service and the date it left. After a month or so you have your own average, which is the only number worth planning against. Somebody else's average belongs to somebody else's load.
Budgeting replacement
If a known share of the fleet drops out monthly, the same share must be bought monthly — a standing line, not an emergency. Counted together with proxies in what a fleet costs.
How to extend it
Lower the load per account and make up the volume with more accounts. That is usually cheaper than squeezing each one dry — method and arithmetic in spreading load and messages per day.
When to take an account out
Before it dies outright: after a first restriction it is worth resting rather than returning it to duty immediately. The rotation order is in rotating accounts.
Buy accountsaged, with full session files