When should a person must apply for the VAT Registration ? 

The answer to this question is within 30 days. This is applicable only for the Mandatory VAT Registration. 

A Person who is required to register for Tax pursuant to the provisions of the UAE VAT Law and rules made there under must file a Tax Registration application with the Authority within (30) days of being required to register. 

The delay in the registration under UAE VAT, when it is mandatorily required under UAE VAT Law and rules made there under, could be subject to financial penalty of AED 20,000/-

Hence the due care must be taken by all the businesses about this legal requirement to protect the business interest. 

Who is a person under UAE VAT ? 
Any  natural or legal person. 
Example: Individuals, Partnership, Companies incorporated in UAE or outside UAE, Companies based in offshore or outside UAE

What is Mandatory VAT Registration ?
All the businesses who have a place of residence in the state of UAE and whose value of supplies in the member states in previous 12 months has exceeded AED 375,000 should mandatorily register under UAE VAT. Also, if the businesses anticipate that the total value of supplies will exceed the mandatory registration threshold of AED 375,000 in the next 30 days, then they too will have to register under UAE VAT.​

Those businesses, who do not have a place of residence in the state of UAE, will have to compulsorily register under VAT irrespective of the registration threshold.

What if person does not register when it is required to register under Mandatory VAT Registration requirements? 
Failure to register when it is mandatorily required to register would attract administrative penalty of AED 20,000/-.