Guitars seem to change faster that violins, probably because guitars are much more lightly built. As an example, a 3mm thick top on a violin is considered pretty normal, while that's seen as 'heavy' for a Classical guitar. The violin is also much smaller, more on the size of a ukulele, and 1.5 mm is more in line for a uke top, so you can see how much more robust the violin tends to be.
In their book on the violins of the Amati family, the Hills wrote a bit about the issue of 'playing in'. They conjectured that the better an instrument sounded the more it would sell for, and the longer the player would retain it. Good instruments were also more likely to end up in the hands of the better players. They used sales records that their family had maintained for several generations to test this. They decided that an 'average' violin by Stainer or Nicolo Amati would play in in about 15-20 years. Strads took a bit longer; 25-30, and in some cases even more, while Guarnari 'Del Gesu' instruments took even longer. Generally speaking the more powerful 'solo' instruments, such as Paganini's Del Gesu 'The Canon', were heavier, and took longer to play in; in that case something like 70 years, iirc. All of this makes sense. Much the same is true of heavier guitars.