Not sure about other banks, but with CBA, I can get them valued whenever I like (at no charge assuming it's as part of a new loan app. or top up). But, you should have a general idea on what the market has done, and there's not much point in getting another valuation done a few months later if the market hasn't moved much.
If you think the valuation should have increased, then go for it, but be aware of what Rixter said with the data delay for valuers. Closest I've had them re-valued is about 3-4 months from memory, but it can be risky. If one of your IP's comes in below the last valuation, the bank could then take that as the new value, and you actually end up worse off.