Its not about length its about whether the term has become so ubiquitous that it identifies a category rather than just a brand. Many, many trademarks have been lost this way and Kleenex is one of those on the verge of being genericized.
https://en.wikipedia.org/wiki/Kleenex > In the USA, the Kleenex name has become, or as a legal matter nearly has become, genericized: the popularity of the product has led to the use of its name to refer to any facial tissue, regardless of the brand. Many dictionaries, including Merriam-Webster and Oxford, now include definitions in their publications defining it as such.
Ironically, one factor is the genericization of trademarks is whether the common usage no longer treats it as a proper noun, that is, whether it is used with or without a capitalized first letter. Your use of the lettering `kleenex` would indicate that it is no longer a brand but just a common noun, and get you a nastygram from the Kleenex marketing team about "Kleenex-brand facial tissues". :-)
https://en.wikipedia.org/wiki/Kleenex > In the USA, the Kleenex name has become, or as a legal matter nearly has become, genericized: the popularity of the product has led to the use of its name to refer to any facial tissue, regardless of the brand. Many dictionaries, including Merriam-Webster and Oxford, now include definitions in their publications defining it as such.
Ironically, one factor is the genericization of trademarks is whether the common usage no longer treats it as a proper noun, that is, whether it is used with or without a capitalized first letter. Your use of the lettering `kleenex` would indicate that it is no longer a brand but just a common noun, and get you a nastygram from the Kleenex marketing team about "Kleenex-brand facial tissues". :-)