for a NDA if a party signed it and sent it by email is that acceptable? and what is the minimum years in terms for an NDA and the maximum

It is valid, but I suggest asking signer to mail signed original to you.There is no set minimum or maximum term for agreement, but I suggest looking at how long you are concerned about keeping the matters undisclosed and then permit agreement to lapse. This is not intended to be legal advice, and is general in  its nature. No attorney-client relationship exists or is formed by this information. Furthermore, this does represent the views or opinions of LexisNexis or its affiliated companies.