Forwarding records in #temp table
The question of when does forwarded records hurt a particular query's performance is too broad to answer, and will vary depending on what the query is actually doing.
Nowhere in the DBA.StackExchange post you linked recommends using a Heap table, rather it shows how to make use of an indexed table by forcing alternative execution plans with proper query hinting to solve the performance problem, when applicable. If you were previously experiencing performance issues that matched the question of that post, I recommend taking up Paul White's advice, but in either case the recommendation would be to index your table, especially if you're joining to it frequently.
Alternatively, Tara Kizer has a great article on How To Fix Forwarded Records which essentially ends with the suggestions of either a temporary fix by rebuilding the table, or a permanent fix by adding a clustered index to it.
And if you're still deadset on keeping it as a Heap then to directly answer your question of &quot;when&quot; to rebuild the table, it generally is recommended to do so based on the Heap fragmentation. That's a more easily measurable number to use to determine when's the right time to rebuild your table. Some people choose to rebuild after 20% fragmentation, others choose 50%, some wait until the table is mostly fragmented, it just depends on your situation and the particular queries you're running against the Heap table. But you can gather more information about Heap fragmentation and rebuilding the table in SQL Server Heaps, and Their Fragmentation.