If the information is available, perhaps in other formats, my understanding is that they can start the challenge immediately, but the formal period for challenge starts after the subsidy is uploaded to the database.
Subsidy Control Bill
Proceeding contribution from
Lord Callanan
(Conservative)
in the House of Lords on Monday, 7 February 2022.
It occurred during Debate on bills
and
Committee proceeding on Subsidy Control Bill.
Type
Proceeding contribution
Reference
818 c346GC 
Session
2021-22
Chamber / Committee
House of Lords Grand Committee
Subjects
Librarians' tools
Timestamp
2022-02-24 13:09:40 +0000
URI
http://hansard.intranet.data.parliament.uk/Lords/2022-02-07/22020762000003
In Indexing
http://indexing.parliament.uk/Content/Edit/1?uri=http://hansard.intranet.data.parliament.uk/Lords/2022-02-07/22020762000003
In Solr
https://search.parliament.uk/claw/solr/?id=http://hansard.intranet.data.parliament.uk/Lords/2022-02-07/22020762000003