Have anyone noticed that trial is good for 7 days?
Does that mean all mined btc will disappear after 7 days?
Or they will wait until I'll buy some hashrates?
They will stay in your account until you buy any contract and then you can withdraw those profits. Only trial will expire and stop generating new Bitcoins
So it's better to keep them for long time without buy as they will mine more in that way...
No, it's the same, they will mine for 7 days either you buy contract or don't. If you buy sooner sooner you will start receiving payouts. Payouts are on hourly basis to your account balance and from there you can withdraw whenever you want and withdraw is instant.