The current regulations (2024 released), fia.com:
1 General conditions
c. An Aerodynamic Testing Period (ATP) is a period of consecutive calendar weeks for the purposes of evaluation of the limits within this Article. As soon as one ATP ends a new one begins, with no gaps between them.
There will be 6 ATPs in any year. The dates of these periods will be as follows:
i. Period 1 will start on 1 January and finish at the end of week 9.
ii. Periods 2, 3 and 5 will run for exactly 8 weeks each.
iii. Period 4 will run for 10 weeks, comprising the Summer Factory Shutdown described in Article 24.1.
iv. Period 6 will end on the 31 December.
2 Restricted Wind Tunnel Testing (RWTT)
RWTT may only be carried out in wind tunnels which have been nominated by the Competitor to the FIA. Each Competitor may nominate only one wind tunnel for use in any one twelve month period and declare it in writing to the FIA. For a new entrant, the nomination must be made no later than 7 days after the date on which it officially becomes a Competitor. No re-nominations may be made for at least 12 months. Nominations should include the facility location, unique identification of the wind tunnel and the scale of model and RATG to be used. The FIA will consider, at its absolute discretion, earlier or temporary nominations if a wind tunnel already nominated by a Competitor suffers a long term failure or for the purpose of evaluating alternative wind tunnels. If a different facility is to be used or if the existing facility is changed or upgraded, other than for routine maintenance or replacement, then a new declaration must be submitted to the FIA within one month of the change or at the time of submission of a testing period report whichever is earlier.
d. In the event of a demonstrated wind tunnel failure or other Force Majeure the FIA will consider, at its absolute discretion, permitting additional occupancy to be used to compensate for that which is lost as a result.
5 Exceptions to the Aerodynamic Testing Restrictions (ATR)
e. Wind tunnel testing that uses a RATG for the sole purpose of the conditioning of wind tunnel infrastructure or the development of wind tunnel infrastructure (including all of its sub-systems such as rolling road, model motion system, force balance, wind tunnel model spine, sensors etc.) and methodology may be performed and will not count towards the accumulation of runs, wind-on time, and occupancy subject to the testing complying with either of the following restrictions:
i. The front wing group and the rear wing group of the RATG must be removed from the wind tunnel for the duration of the testing. Alternatively, either one of or both the wing groups may be retained on the model, but each that remains must be fitted with a bluff cover that has been approved for this purpose by the FIA. The front and rear wing groups will be considered to be bodywork described by Articles 3.9 and 3.10 of the Technical Regulations respectively.
ii. A RATG is used which is more than 12 months old, or represents an FIA approved CAD geometry provided for this purpose and that no modification is made to this previously tested RATG or FIA approved geometry.
They can nominate the new wind tunnel once it's ready. Until then they obviously use the Brackley tunnel.
The only way to have more wind tunnel time later is if a failure appears, which is not the case at AMR.
And at the last part the exceptions to the ATR (I guess that's what they are doing in Silverstone since months).