You can use the free version of Slack for as long as you'd like, and there's no limit to the number of members who can be invited, but there are a few usage limits to consider:

• Message visibility
  Only the most recent 10,000 messages can be viewed and searched.
• File storage
  Add up to 5GB of files to your workspace.
• Apps
  Install up to 10 third-party or custom apps.

Tip: Upgrading to a paid plan will remove message and app limits, and increase your workspace's file storage limit. 

 

Message visibility limit

Members can send unlimited messages in Slack, but only the most recent 10,000 can be searched and viewed in your workspace. Keep in mind that you'll only be able to see messages in public channels, private channels you belong to, and your direct messages when searching. 

If your workspace exceeds the message visibility limit, members can continue to send messages, but older messages will be archived. If you upgrade your workspace to a paid plan, all archived messages will become available again.

What counts as a message

Anything typed into the message field and sent to a channel or direct message counts as a message. A one-word message or single emoji is counted the same as a message with 500 words and multiple paragraphs.

Links, snippets, posts, uploaded files, replies in a thread, messages in archived channels, and messages from bot users all count towards your workspace's total number of messages sent.

Note: Deleted messages do not count towards the message limit.

 

File storage limit

On the free version of Slack, workspaces have up to 5GB of file storage available. Anything uploaded to a channel or a direct message (e.g., PDFs, documents, images, screenshots, audio and video files) counts towards this limit.

If your workspace reaches the limit, members can continue to upload files, but Slack will start archiving the oldest files to make room for new ones. Members won't be able to view archived files in channels, direct messages, or search results.

Like messages, archived files will immediately become available again if you upgrade to a paid plan. If you choose to upgrade, file storage limits will be determined by the total number of members (including Multi-Channel Guests) in your workspace:

• 10GB per member on the Standard plan
• 20GB per member on the Plus plan 

Note: Deleted files and files added to Slack via apps (like Google Drive, Dropbox, etc.) do not count towards the file storage limit.

App installation limit

Members of workspaces on the free version of Slack can add up to 10 apps. Apps from the Slack App Directory and custom apps built for your team count towards this limit. 

If your workspace reaches the app installation limit, you can uninstall apps to make space for new ones, or upgrade to a paid plan to install as many apps as you'd like.