The restrictions in the demo version are described on Pricing page. You can use the demo version without a valid license. There is no time limit, the demo version never expires. No watermark on invoices and no nag screens only one f-billing branding line on invoices. You can anytime buy a valid license. Your license key will immediately upgrade the demo version and turn it into the full absolutely unlimited version. No need to download and install again after registration. All your existing data and configuration settings will remain exactly the same!