If I pay three~five times - my payments will be accumulated?
I mean - can I pay less than 0,1ETH right now (I've paid more in first day). Or such small amount will be rejected by contract?
Sure, You can. But all the transactions must be from the same address. The sum of all transactions by the end of ICO must be greater than 0.1 ETH. If it'll be smaller - funds will be returned to the senders address.