This is from the dialogue titled Sophist, the Fowler translation at perseus.tufts.edu gives it as:
No one should be discouraged, Theaetetus, who can make constant progress, even though it be slow.
And the Jowett translation at Project Gutenberg translates it as:
Any one, Theaetetus, who is able to advance even a little ought to be of good cheer
And in the Nicholas P. White translation in Hackett Publishing's Plato: Complete Works, it's translated on p. 285 as:
Even if you can only make a little progress, Theaetetus, you should cheer up.
According to the perseus.tufts.edu page, this line is from location 261b in the text, the corresponding Greek text can be seen here.