You can book hotels or timeshares approximately 50 weeks in advance, if Rewards are made available. Marriott adds inventory that far in advance very early every Sunday morning. You can prove it by going to Marriott.com and pretending to try for a hotel (not a timeshare) reservation in late December this year. You'll quickly see how far in advance you can reserve. Inventory, if made available is added at the same time for hotels and for timeshares.
However, don't count on booking at a timeshare with points, especially at Newport Coast in June. Until the last few days, there has rarely been Reward stays available at most Marriott timeshares, especially in peak periods such as you seek. The new no-blackout rules for Reward stays will apply to hotels, but not to timeshares.
So hope for the best, but be sure to have a back-up plan.