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I just got some 'instant mold' in the post. I got it from an ebay dealer so 10 blocks for £11, but the blocks look a bit small to me, does anyone know the size of the branded 'instant mold' stuff?

and I watched a video showing that you can re-use it by heating it up again, does anyone have any experience on how many times you can re-use the stuff?

ps. I feel dirty not spelling it 'mould' but it is an American invention so I'll just have to put up with it....

Just because it was made in America doesn't mean its patented the word Mould. :p (Google chrome seems to think that's not a word xD)
Ive used a little before, depending on what you want to cast in it is what will make it come out better or worse.

The blocks are small (off the top of my head each one is smaller than my little finger) but can go a long way. I have re-used it quite a few times without problem. Just make sure you wash it after casting to ensure you don't trap too many impurities in the mould material.

Sometimes after casting something with greenstuff or instant putty, some of the putty is left in the instan-mold. As long as you clean that stuff out of there to keep the intant-mold clean, there is almost no limit to how many times you can use it.

and yes, the blocks are pretty small, Not enough to copy a full model, but more than enough to copy a few bits.

Instant mold is just a rebranded (and marked up) product which is variably sold under names like Friendly Plastic, Oyumaru, InstaMorph, ShapeLock and a few dozen other names. It is most likely a chemical compound like polycaprolactone - though to be honest...I haven't spent that much time looking into it (that is the most common plastic that I know of - though there are a few others with comparable properties).

The "blocks" of the more common brands which are sold in that manner (as opposed to pellet form) are about the size of a stick of gum. Pretty small. This is in order to aid in even heating without overheating.

In terms of how many uses you will get out of it, it will depend on a number of factors. The secret is within the plasticizers within the compound and the molecule itself. Repeated heating does "cook out" the plasticizers and break down the molecules. The hotter it gets, the more damage which is done. Light, especially UV light will also be detrimental - so ideally you will want to keep it in the dark when not in use.

When you get ready to reuse the plastic, you will generally want to break up any chunks to make them small and uniform. As with the original packaging - the small parts will allow the plastic to heat quickly and at the same rate throughout the batch. If one portion heats up faster than the rest, that portion runs the risk of overheating and being damaged faster than the rest.

It will also break down on its own, and does have a shelf life even if you never use it or expose it to light. The molecules oxidize and begin to fall apart from day one. According to one of my big books on polymers, it has a projected useful lifespan of roughly 5 years.

Most of the time I get about a years worth of work before I buy more - though I probably use it more than most people. I buy mine in 20 ounce containers ($30 each).

Roughly the size(little smaller) of a pack of thin chewing gum (Juicy Fruit and such). Each package it comes in has about 3-4 of those.
I have used it for a year now, and didn't need more than 1/10 of the whole package(though I don't try to mould whole models).