The normal level is zero, and in many cases the legal level is zero. You have a duty of loyalty (“obligation de loyauté”) to your current employer. The law does not define this precisely — in fact it doesn't even use this expression except in a secondary clause, but there is considerable jurisprudence.
I'm no expert, so I won't venture to say what this covers exactly. But if your former and current employers are competitors, you should definitely not be helping your former employer at all. Perhaps less obviously, you should not help your former employer if they're potential clients of your current employer, because then you would be competing as a vendor of services against your current employer. Obviously you should be careful not to reveal any confidential information that belongs to your current employer, even if they aren't competing at all. Basically you need to be careful not to run into any potential conflict of interests.
This duty does not prevent you from being paid by your former employer. What matters is the nature of what you do and not whether you're paid or not. If you have a full-time job, your contract may include an exclusivity clause (“clause d'exclusivité” — unlike the duty of loyalty which is public policy, a duty exclusivity can only arise from a contract). An exclusivity clause forbids taking a second job or having regular activity as an independent contractor, but I think it does not preclude occasional contractual work (and there's also a loophole if you become auto-entrepreneur).
So tread carefully. It's unlikely that “the X document is in the Y folder” would ever be problematic, but ideas on how to solve issues are another thing. In any case, it is not normal for a former employer to ask for new ideas. If they want your expertise, they should offer to hire your services — which, as I explained above, you may not be permitted to provide (depending on the nature of the activity).