The dividends are paid every 6 months and the ADR issuer will announce the date just like a normal dividend (may be slightly later than the Japanese share date). The dividend in December was about 86 cents/share, but this can obviously vary quite a bit based on profits and exchange rates - like many foreign companies, Nintendo targets a percentage of yen-denominated profits rather than just paying a fixed amount each quarter.
The ADR issuer will withhold and pay 8% foreign tax on your behalf. In an IRA this money is a loss because there's no corresponding US tax to cancel out (i.e. that part of your IRA income is now foreign-taxable and the IRS doesn't care because it's an IRA). In a standard account you can take a credit against your taxes for foreign tax paid.
Either way you do ok, because this is a pretty low withholding level -Europeans other than UK often take out 35%. UK is just about the only major country that doesn't withhold taxes on dividends to US holders. The ADR issuer will also charge you a small fee, around 2-3 cents/share, in exchange for passing through the dividend in dollars. Otherwise I think there is no fee on the ADR (they don't take anything off the purchase/sale of shares or charge a holding fee). Overall, holding Japanese ADRs definitely beats owning a mutual fund with excessive expenses.