When can I expect to receive my tax refund?
Average timeframe for the foreign tax office to deal with tax returns is up to a year. Especially M forms and C forms take a lot of time. An M form is filed when you were not in the Netherlands the whole year, a C form is filed when you were living outside the Netherlands but had income or assets in the Netherlands. For the period between the end of the tax year and the day you will receive the tax assessment the tax authorities will pay you interest.
First you get a provisional tax assessment and after a while a final tax assessment. So receiving a provisional tax assessment (voorlopige aanslag) doesn't mean that the tax authorities agree with your tax return. They just pay you the money.
Unfortunately the foreign tax office often doesn't cooperate if they are requested to just issue a provisional assessment in advance so that a refund can be received earlier.
If you would like to know the status you can always call the international helpline of the tax authorities on 0031 (0) 55-538 53 85. However, the only thing they can see in their system is whether a tax return has been received, but they have no idea when it will be processed, unless a tax assessment will be send to you on short notice.