I've always been under the impression that agents/landlords had to give 24 hours' notice before entering the property... actually, I'm pretty sure that stipulation is written into the contract for our flat.
We gave two months' notice on our flat back at the end of July, just before I went on holiday for two weeks. While I was away, I had one text message from the agency asking if it was convenient for them to come for a viewing the next day and one voicemail from them again a few days later regarding another viewing. When I returned from holiday, one of my flatmates informed me that we'd had 8 viewings in total and that they'd found new tenants already. I'm assuming that the agency also contacted my flatmates about the other viewings before showing up at the flat, but I only had two messages from them while I was in France.