The normal deadline for filing is before April 30 of the year following the concerning tax year (calendar year). However, there are a few things that are good to know.

Deadline in case invited to file a return ("Aangiftebrief")
In case you were invited to file an income tax return (if you received an "Aangiftebrief"), the deadline will be mentioned in the letter - no matter what the expected result of your return is. Usually this invite is sent early in the year (January or February), sometimes however, it is send in September or October. In that case a different deadline will be in the letter. 

When you miss the deadline for filing, you will first receive a letter called "Herinnering" (reminder) with a new deadline, next in case you did not make the new deadline, you will receive a letter called "Aanmaning" (urgent reminder) with a final deadline. Once the final deadline expired a penalty for not filing (timely) will be added to your tax assessment. 

If you do not file at all, it is possible that the Dutch tax authorities isseu an assessment on the basis of an estimation - which is in general a very unbeneficial estimation. They will also include the penalty for not filing (timely) which amounts to 

Extension for filing
It is possible to apply for extension for filing. If you apply as a private person, the Dutch tax authorities usually provide an extension until September 30 of the year following the concerning tax year. Extension for filing for tax consultants is usually an extension until April 30 of the year after the normal filing deadline (so 1 year of extension).

If you file later than April 30
If you did not receive the letter called "Aangiftebrief" and/or if extension of filing was applied for, you can file your Dutch personal income tax return later than April 30. However, in such situation the Dutch tax authorities will charge interest on top of the amount of payable tax, in case they issue the (preliminary) tax assessment later than July 1 of the year following the concerning tax year.

If the tax return results in a tax refund, the Dutch tax authorities will refund interest on top of your tax refund if they fail to issue an assessment within 3 months from filing of your income tax return.