How long does a good application last?

I am not sure at this time. I bought my car at the end of March 2013. I treated the top and then a month later I treated it again just to make sure it was thoroughly covered. It still looks brand new today. Water beads up on, it and I will see just how long it last this time before I treat it again.

What is the difference between this and scotch gard fabric protector?

Scotch guard is basic interior protection for furniture fabric. An expensive canvas convertible top requires protection from the outdoor elements including snow and rain. Ragg top is the only product endorsed by canvas convertible top manufacturers wether the top is on a Rolls or a Fiat. Scotch guard has no such endorsement because it is not intended to be used on convertible tops and is mediocre at best in protecting fabric. I Live in the snow belt with harsh winters and have used it on my Solara Convertible top since the car was purchased new in 2007. The top (black) still has a new appearance and I am often asked what do I apply to maintain the appearance, when you see the faded ,stained tops you can bet they aren’t using Ragg top but then again they are saving $16 a year. Common sense isn’t too common.

Can this be applied on top of 303 brand canvas top protectant that was applied 3 months ago?

Why not….you’ll get even better protection…..

can this be used on neoprene

I don't believe it can. There is a specific product for vinyl and this one for cloth (fabric) . My roof is fabric (sail cloth) and it works great on it.