The bulk download attachments extension has no maximum limit. The download time depends entirely on your Internet connection speed.
If you're having issues like this, you can try the following:
1. Divide the records
Break down your table into multiple batches using the view filter option and trigger the download for each view separately. The lower file size might increase the success rate of downloading the files.
2. Use the Airtable extension
Another alternative you can try is to use our Marketplace extension: https://airtable.com/marketplace/blk6Ej3T3Azbui6ee/bulk-download-attachments
3. Rename attachment files
If some of your files are missing their extensions (e.g .pdf), try renaming your attachments first. https://miniextensions.com/rename-attachment-with-airtable-formula/