This really depends on what accent you speak, what your vocabulary is like, where you are traveling to, and what sort of person you plan to interact with. And just how much would you want to "fit in": is it enough to pass for someone from some English-speaking place the people you're talking to haven't been? For my part, I'll explore how you might do in the 1800s.

Pronunciation is important. Do you speak with a rhotic r, as people in the United States tend to do and people in England generally don't? In other words, do you say "pay-per" (rhotic) or "pay-pah" (non-rhotic)? The rhotic is older, and would get you more mileage. The non-rhotic variation only started to be common in the 18th century, though some r's have been softening since the 16th. So if you're from a "pay-pah" part of the world, you'd be limited to about 200 years, depending on what specific accent you have. If you happen to speak with a rhotic English (as in, from England) accent, such as they still do in parts of southwest England, that would be your best bet, as that would push you back through the 1700s.

Or do you speak with the General American accent? This specific accent has, to the best of my knowledge, only been dated back to the start of the 20th century, so 100 years for you. Of course, speaking with a different accent won't make you sound like an unintelligent hobo, but it would mark you as not being from the same area/group. If you would like to hear a little of how people in the United States sounded who grew up from the mid-1800s, I recommend these interviews. I know that, for me, understanding them isn't difficult, but there is a clear difference in pronunciation with the US accents I know from today.

Another important question: what kinds of people are you planning to speak with? Class differences in vocabulary are very prominent. What do you call the playing card marked with a J? If you call it a Jack, an educated person might respond like Estella does in Great Expectations (1860): "He calls the knaves, Jacks, this boy!... And what coarse hands he has! And what thick boots!" To add to that, could you sprinkle your language with expressions in French, and use Latinate terms? Would you be baffled when someone describes something as charmant, and would you know what you were looking for when asked to locate a pullet lost in the verdure? (hint: follow the clucking)

As for talking to the working class, you might struggle there, too. Here's one of my favorite scenes in Oliver Twist (1837), in which Oliver fails to understand Dodger's reference to a "beak":

'My eyes, how green!' exclaimed the young gentleman. 'Why, a beak's a madgst'rate; and when you walk by a beak's order, it's not straight forerd, but always agoing up, and niver a coming down agin. Was you never on the mill?'

'What mill?' inquired Oliver.

'What mill! Why, the mill—the mill as takes up so little room that it'll work inside a Stone Jug; and always goes better when the wind's low with people, than when it's high; acos then they can't get workmen. But come,' said the young gentleman; 'you want grub, and you shall have it. I'm at low-water-mark myself—only one bob and a magpie; but, as far as it goes, I'll fork out and stump. Up with you on your pins. There! Now then! 'Morrice!'

Dickens has been accused of making up some of these expressions, but a quick perusal of a dictionary of Victorian slang will probably leave you scratching your head.

What about your grammar? Do you say "one" to refer to a general person, or "this is wanting" when something is missing? (both of which are falling out of use, but were common in the 19th century) Would you use "for" to mean "because?" (the latter started squeezing out the former around 1900) If not, you might stand out. Do you use the progressive passive (it was new in the early 1800s!)? In other words, would you describe a bridge-in-progress as "building" or (the newer) "being built?" If nothing else gave me away, this definitely would.

What about your modern vocabulary? Would you ask your friend "What's up?" or inquire about where they will be "hanging out?" ... actually, these expressions existed at least as far back as the Victorian era, though other modern expressions might well get you spotted! "Cool", meaning trendy or generally good, has been dated to the 1920s.

Then there is the simple shift in technology and the language that goes with it. Do you know what an andiron or a firedog is? (They're the same thing--an iron stand for holding firewood in a fireplace.) What about a chatelaine? (A housekeeper's belt clasp for hanging utensils and keys from on chains.) A pince-nez? (Glasses with no stems perched on the nose.)

To be able to give you a clearer picture, I'd have to know more about how you speak. I would wager your vocabulary and grammar would trip you up before your pronunciation did. For my money, I'd say between 100 and 200 years, depending on where you're from and your grammar/vocabulary.

This question gets asked here a lot. Not discouraging further answers, but this section of our FAQ may be of interest.