Iron will oxidize at all levels. Depending on your treatment (especially chlorination) it may take different lengths of time to oxidize. What you probably mean to ask is at what level will it pose an aesthetic problem. Iron has no maximum contaminant level (MCL) because it is not harmful to the health; but, it does have a secondary standard (SS) because of its aesthetic qualities. It turns the water red and stains clothes, sinks, dishwashers, toilets, tubs, etc.
The SS for iron is 0.3 mg/l. I have found that in most cases, if the level of iron exceeds this, it does in fact cause concerns with users fairly rapidly. Even at lower levels in a system, some iron settles out of the water and deposits on the bottom of the pipes. During a pipe break or high flow, some of these deposits can be lifted and carried down the pipe where it causes problems. It can take time for this to occur at lower levels.
There are several methods for removing iron and a search of the threads in this forum will reveal a lot of information. You might also check the manganese levels as often times it can be present as well.
