My wife is a South African citizen and has been married to me (I have a Dutch passport) for 30 years. How can she obtain a Dutch passport? Both my daughters have Dutch passports. If not, how can she apply for a long term Schengen visa?
She cannot apply for a Dutch passport without first becoming a Dutch citizen.
In general, one must live in the Kingdom of the Netherlands to be naturalized, but as the spouse of a Dutch citizen, she can apply for naturalization even though she lives outside the Netherlands:
In the following situations the 5-year term [of residence in the kingdom] does not apply.
You are married to or are the registered partner of a Dutch citizen. After 3 years of marriage or registered partnership and having uninterruptedly lived together during that period, you can submit an application. These 3 years may also have been spent abroad. You must be living together during the whole procedure.
Also, in general, someone naturalizing as Dutch must renounce any other citizenship. In this instance as well, your wife benefits from an exception:
There are various situations in which you do not have to renounce your nationality. You do not, for example, have to give up your nationality in one of the following situations:
You are married to or are the registered partner of a Dutch citizen.
There are several other requirements she must meet, including passing the civic integration examination.
If you wish to move to the Netherlands together before she naturalizes, you can sponsor her for a residence permit.
If she wants a long-term visa for successive visits to the Netherlands, she may be out of luck; all she can do is apply for a regular Schengen visa and ask for a long-term visa with multiple entries. She might get lucky.
If you wish to visit another Schengen country with her, however, she can apply as the family member of an EU citizen, in which case her visa will be free of charge and should be issued expeditiously. The EU right of free movement of persons includes a "right of residence for up to three months."