The recommended time frame is ASAP. Subsequent closings will depend on demand - however once you do a 1st close most LPAs specify that you have 18 months to close the rest of the fund (often called the Fundraising Period). Depending on the LPA, this can be extended, however this isn't always an option, and may not be strategically a good move. 

TL:DR - don't rely on extending the Fundraising Period if possible.  

Decile Partners and VC Lab strongly recommend continuing to prioritize fundraising (~80% of a fund manager's time and focus) until the entire fund has been closed.