It depends a lot on your standards. I'd say that an 1800 ppi scan looked bad at any more than 6x enlargement, so an A2 from 35 mm would show what I would describe as 'noticeable degradation'. As a rule of thumb, you can divide the scan resolution by 300 to arrive at a very approximate value for the degree of enlargement you can get before there is some degradation. It depends a lot on the nature of the image and the post processing techniques as much as on the capabilities of the scanner or the film itself.
For example, some Epson inkjet printers do a very good job when printing a file that is sent to them at 288 ppi (it is a native resolution of the printer when the printer itself is set to 1440 or 2880 dpi quality), so a scan made at 4000 ppi can be enlarged almost 14x without any interpolation.
Best,
Helen