So the question is simple. How many light years would we have to go from Earth until we would be unable to determine known constellations?
Well, we can determine positions of galaxies in a very large region of Universe with precision of about 1%. So if we're talking about relative accuracy (bigger error is acceptable on bigger distance from Earth), then in static Universe we probably would be able to venture billions of light-years and still know our position relatively to Earth up to 10% error margin. Finding approximate direction to Earth is even easier.
Unfortunately real Universe is moving and evolving, so probably if you travel for billion of light-years it will change too much for existing maps to be still useful for navigation. So we should restrict ourselves a bit in time. Still you can likely go for dozen millions of light-years and will still approximately know where you are.
Surprisingly navigating within our own galaxy might be more complex. We need stars for navigation with good accuracy on a galactic scale, but interstellar gas and dust clouds block large regions of our galaxy from our sight. So I'd say we need to go to probably 40 million of light years in specific direction (towards Milky Way center) to get lost. You might get lost even closer if you'll end inside of sufficiently dense gas or dust cloud that will block even IR light (but it's hard to find and maybe we still would be able to see some stars with radio astronomy even in that case).
Like before, time is also an important factor. Wait long enough and your maps will became obsolete even if you hasn't moved far away. Stars are changing their positions far faster than galaxies so even few hundreds thousands of years will present a challenge.
Of course navigation in all cases should be based on modern star maps like one that Gaia satellite produces rather than "constellations".