My reading of this has always been that they practised Ron's disguise thoroughly, before they actually attempted the break-in. So I guess Hermione had done a few experiments at Shell Cottage and the most recent one is the "last time" Ron refers to here.
We know from the previous chapter that their plans were meticulous and that they took the attack very seriously:
It was like planning to break into the Ministry all over again. They settled to work in the smallest bedroom, which was kept, according to Griphook's preference, in semi-darkness.
They remained shut in the cupboard-like room for hours at a time. Slowly, the days stretched into weeks. There was problem after problem to overcome, not least of which was that their store of Polyjuice Potion was greatly depleted.
'That'll be enough,' said Harry, who was examining Griphook's hand-drawn map of the deepest passageways.
The other inhabitants of Shell Cottage could hardly fail to notice that something was going on now that Harry, Ron and Hermione only emerged for mealtimes.
Harry Potter and the Deathly Hallows - pp.411-2 - Bloomsbury - Chapter 25, Shell Cottage
And after all:
'...yeh'd be mad ter try an' rob it, I'll tell yeh that. Never mess with goblins, Harry. Gringotts is the safest place in the world fer anything yeh want ter keep safe - 'cept maybe Hogwarts.'
Harry Potter and the Philosopher's Stone - p.50 - Bloomsbury - Chapter 5, Diagon Alley
So, all in all, given that:
He was to be given a completely fake identity, and they were trusting to the malevolent aura cast by Bellatrix to protect him.
Harry Potter and the Deathly Hallows - p.422 - Bloomsbury - Chapter 26, Gringotts
It seems almost inconceivable that they wouldn't have practised this - as you would, surely, in the Muggle world. Not that I have any personal experience of bank jobs, I hasten to add! But I'm sure you'd try your disguise out before the big day. And human Transfiguration is quite difficult:
'If only we'd done human Transfiguration already! But I don't think we start that until sixth year, and it can go badly wrong if you don't know what you're doing...'
Harry Potter and the Goblet of Fire - p.418 - Bloomsbury - Chapter 26, The Second Task
Also, remember that scene in Harry Potter and the Half-Blood Prince, where Ron gives himself a handlebar moustache!
They had just embarked upon the immensely difficult topic of human transfiguration; working in front of mirrors, they were supposed to be changing the colour of their own eyebrows. Hermione laughed unkindly at Ron's disastrous first attempt, during which he somehow managed to give himself a spectacular handlebar moustache...
Harry Potter and the Half-Blood Prince - p.290 - Bloomsbury - Chapter 15, The Unbreakable Vow
So I'm quite confident that the trio would have experimented with various different looks for Ron and that that's what Ron means. Like you, I can't find any other reference to Hermione tweaking aspects of Ron's appearance using Transfiguration spells.
And even if she had, the context really makes it sound to me like Ron's referring to what makes a good look for his disguise as Dragomir Despard and there aren't any other instances of him assuming such a disguise in this way - they've always used Polyjuice Potion. Also, when he says "remember, I don't like the beard too long" - again, it sounds like they've been practising this recently.