“5-min” video vs “5 min” video vs “5min” video

Only the hyphen is correct. The two (or more) words combined form a compound adjective to describe one aspect of the video. Note the singular form of the full word (five-minute video - not five minutes video) despite the plural number: this is the clue a hyphen is needed, although the rule applies of all compound adjectives.

Consider:

• 14-year-old boy
• six-foot wall
• long-lost relative
• all-in-one printer (actually printer/scanner/copier and, sometimes, fax)

There should be a space between the value and the unit.

Source: NIST - Rules and Style Conventions for Expressing Values of Quantities

Edit: Hyphens should only be used if the symbol for the unit is not used.

Occasionally, a value is used in a descriptive or literary manner and it is fitting to use the spelledout name of the unit rather than its symbol. Thus, this Guide considers acceptable statements such as “the reading lamp was designed to take two 60-watt light bulbs,” or “the rocket journeyed uneventfully across 380 000 kilometers of space,” or “they bought a roll of 35-millimeter film for their camera.”