WI the Netherlands joined the CP? Probably a bad idea because they will lose the DEI...
They probably would, but the place is big and has plenty of ships around. If Japan goes all-in on it it could be done quick, but if they stay out and Britain/France have to scrounge up the troops and ships to take it from somewhere it'd take longer.
The real positive is it might soak up enough resources to make Gallipolli impossible
If the Dutch manage to surprise the Entente by joining the CP right in 1914 (butterflies be dead), the combined Dutch-German offensive could run all the way to Calais-Paris before running out of steam, putting the German-Dutch navies in a pretty scary spot for the British, and making their logistics terrible. More of the Belgian army would be destroyed rather than pushed away, too (what with the Dutch army starting practically in spitting distance of Antwerp, with good railways to bring in big German guns). They'll be able to win the war before the DEI actually fall, I'd say.
If the Dutch join the CP late in 1914, or with the Entente able to predict it weeks if not months in advance, the net effect is probably still good for the CP but much less so, and you might see the DEI fall at some point (1917?), while the lack of a Dutch neutral outlet makes the blockade tougher on Germany.