It's always tricky to discuss english style because it can be so varied and depend on demographics, age, generation, etc. But to my ear, 'for' only sounds right when followed by a noun as in, 'How much time for lunch'. With a verb, I think it should be 'to replace'. But hey, that's only my opinion. :)
Not sure how your comment relates to my reply, nevertheless I disagree.
Your proposal (from the About.com website) is an example of forming the present participle from the the present tense. The site also emphasised that this construction denotes an action taking place "right now".
"... infinitive in French does not always translate directly to the infinitive in English."
The infinitive English verb is "To play/Play". The example you used - "...en train de jouer" - isn't an example of the French infinitive.
As for my initial reply, is the intention in the sentence to reflect an action - i.e. to replace something? A verb's used as a gerund when it has a noun function.
p.s. I always to stand to be corrected if others are willing to provide a good explanation.
I understand what the infinitive is in English.
The point of my comment is that the French infinitive, as well as other words, may not have to be translated directly to their English equivalents. I just wanted to bring this to your attention since you initially commented that "remplacer" is the infinitive form. Additionally, there is usually leeway in translation since certain styles may be used in one region but not another. So, while translating the question as "How much time for replacing her?" may seem correct to another user, it seems incorrect to you.
"the French infinitive... may not have to be translated directly to their English equivalents"
This isn't saying much tbh wasn't really relevant to my reply to the OP. It seemed he was saying "remplacer" here could take the gerundive form. Whereas, imo, there's a clear delineation in the phrase's meaning.
Simply put, there's no "leeway" needed here. I'm not making a general comparison between the two languages - I'm too much of a French novice for that. I'm saying its either a verb or a gerund, not both.
The infinitive follows a preposition and requires an action. That seems clear to me. Therefore I would not translate this as a gerund.