galvanized pipes - when to replace

[Peter Grote]

I am starting to purchase SFR homes to buy and hold. The homes I'm getting tend to be older 1900-1950 and so have galvanized pipes. What rule of thumb do people tend to use for when to replace them? when something's leaking? or when the hot water flow is becoming too slow? or when it's convenient because you're doing other rehab anyway? and then do a little at a time or all at once? thanks