Depends on how fast your grass is growing. Iron only affects the portion of the blade it touches, so as you mow that section off and expose new growth, it's safe to spray again.

Moderation is a virtue. Here's what I found on iron toxicity (Carrow's soil fertility book): "High foliar Fe can blacken leaves and, if sufficiently excessive, cause tissue injury; can induce Mn deficiency; acidic, poorly drained soils can produce toxic levels of soluble Fe for roots."

Is there a reason you want to spray vs. spread granular iron? I think spraying is labor intensive and transient that it's not really worth the effort.

It’s just what I have on hand. What brand do you recommend and how much of a granular?

Try to find a chelated form. If you have high pH, nonchelated forms of iron can get tied up (unavailable) in the soil. One of the reasons Milorganite is popular is because it acts like a chelated form of iron.

+1. the reduction in iron content of the new milorganite is a travesty. i no longer notice the lawn getting darker green after application.

I noticed that too