As with all high quality internal silicone sealants, 3C HM has great movement accommodation with a capability of +25% in accordance with the industry standard ISO 11600. In layman's terms this means that the fully cured silicone sealant will comfortably expand (and contract) within 25% of it's original width to allow for movement in the substrates making it perfect for bathrooms, shower rooms and kitchen areas in particular.
When formulating this internal silicone sealant, we used our experience and know how to find the 'happy medium' for a product that guns out with good body (eg. it's not too runny) but also not too stiff. The many different HM products in the marketplace vary greatly when it comes to the consistency of the wet material and, although it's largely down to personal preference, we carried out extensive market research to discover the sweet spot for perfect application (gunning out) and finishing (tooling off) and 3C HM was what we settled on. Judging by the sales feedback, we reckon we got it just right!
We don't recommend it! There's often confusion regarding the categorisation of mastic joint sealants with regards to what should be used where and the truth of the matter is that there's science-backed sensible reasons why certain types should be used indoors and other types outdoors but without putting our lab coats on and boring the death out of you, we can explain why simply with 2 x words... Curing and Movement. Thus any LM (or Low Modulus) should be considered as an external product and any HM (or High Modulus) should be considered as an internal product. These are basic principles that should be adhered to when deciding where to put what.
This is a tough question to answer outright, as there's many variables that determine the longevity of the final cured sealant. Providing the surfaces were clean, dry and free from dust & dirt etc. and a good adhesion was achieved, then the 2 x most important factors to help keep your sealant looking fresh and mould-free is to make sure you clean your substrate after use and perhaps even more importantly that you have adequate ventilation in the room. In general, a premium quality sanitary silicone sealant such as 3C HM should last anywhere from to 10-20 x years with proper care and attention.