Here are the rules from Hilton website:
Hilton Rollover Nights FAQ
9. How can I find out how many elite rollover nights will apply to the next status year?
To calculate the number of elite rollover nights you have achieved to-date, you can apply the following formula:
Total qualifying nights at year end (inclusive of rollover from prior year) — any elite rollover nights from the previous year (now expired) — qualifying nights needed to renew your current elite status tier = elite rollover nights for next year.
Example: You rolled over 5 nights from 2018, you stayed 45 nights in 2019 and gained Gold Status. The calculation for 2019 rollover nights would be as follows: 5 rollover nights from 2018 + 45 nights from stays in 2019 — 40 nights needed for Gold — 5 rollover nights from 2018 (now expired) = 5 elite rollover nights into 2020
These elite rollover nights will be automatically calculated for you and applied to your member account at the beginning of the next calendar year — no action is required from you.
Diamond members with 120+ nights within a calendar year will rollover 60 nights into the new year, as 60 nights will ensure you maintain your Diamond status for another year.
10. For how long are my elite rollover nights valid and when can I apply them toward status? What happened to my elite rollover nights from last year?
Elite rollover nights are only valid for the next calendar year and expire thereafter. Elite rollover nights cannot be reserved and applied to a second, third year or subsequent year.
Example: You rolled over 10 nights from 2018, you stayed 20 nights in 2019 and gained Silver status. The calculation for 2019 rollover nights would be as follows: 10 rollover nights from 2018 + 20 nights from stays in 2019 — 10 nights needed for Silver — 10 rollover nights from 2018 (now expired) = 10 elite rollover nights into 2020Elite rollover nights must be used in the following calendar year from when they were earned. In this example, the 10 elite rollover nights from 2018 must be used in 2019 and cannot be applied to the 2020 calendar year.
11. How many elite rollover nights can I carry over into the next calendar year? T
here is no limit; however, you'll only roll over nights in excess of those needed to earn your elite status level in a calendar year.
While there is no limit, Diamond members with 120+ nights will only see 60 rollover nights in their trackers, because only 60 rollover nights are needed to ensure you maintain your Diamond status for another year.