How do I know which one to use? Could I use both of them interchangeably all the time?
I think it would be so much for asking/to ask
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I think these things are difficult, partly because whatever grammatical rules might apply can get pretty technical, and native speakers do not know the rules in their language anyway -- we go by what 'sounds right' without getting bogged down in the rules. That doesn't mean they aren't there, just that, while forming a sentence, no one thinks thought noun-verb-direct object, much less gerunds, etc.
"for asking" sounds like something that happens because someone asked something. That form of the sentence could actually apply if, for instance, someone asked for a drink, and someone else hit them in the face -- "That seems so much for asking", meaning that it seems like an overreaction to a request for a drink.
Assuming that you mean "what I am asking seems like a big thing to be asking for", then that would be "I think it would be so much to ask". "To ask" is the action of asking (unlike 'for asking'), and it would be understood to be the shorter form of "to be asking for". Maybe that's the way to remember it; it's a shortened form of a longer phrase, and the verb form doesn't change in the shorter form.
I think it would be so mmuch to ask. I think it would be so much to be asking for.
I think it would be so much to ask for.
Hope that helps.