Event the people who find that caffeine gives them a "stress response" or raises their blood pressure, cortisol, etc should be able to quickly adapt. This study shows complete adaptation to all parameters of the "stress response" induced by 250mg caffeine daily. The adaptation occurred in as little as 1 day and by day 4 all subjects had adapted and no longer produced a stress response to caffeine, while the beneficial metabolic effects were still present.
Doses higher than 250mg may require up to a week, but I saw studies showing full tolerance to doses as high as 2g caffeine per day.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC370671/
"...Acute caffeine in subjects who do not normally ingest methylxanthines leads to increases in blood pressure, heart rate, plasma epinephrine, plasma norepinephrine, plasma renin activity, and urinary catecholamines. Using a double-blind design, the effects of chronic caffeine administration on these same variables were assessed. Near complete tolerance, in terms of both humoral and hemodynamic variables, developed over the first 1-4 d of caffeine. No long-term effects of caffeine on blood pressure, heart rate, plasma renin activity, plasma catecholamines, or urinary catecholamines could be demonstrated. Discontinuation of caffeine ingestion after 7 d of administration did not result in a detectable withdrawal phenomenon relating to any of the variables assessed."
Doses higher than 250mg may require up to a week, but I saw studies showing full tolerance to doses as high as 2g caffeine per day.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC370671/
"...Acute caffeine in subjects who do not normally ingest methylxanthines leads to increases in blood pressure, heart rate, plasma epinephrine, plasma norepinephrine, plasma renin activity, and urinary catecholamines. Using a double-blind design, the effects of chronic caffeine administration on these same variables were assessed. Near complete tolerance, in terms of both humoral and hemodynamic variables, developed over the first 1-4 d of caffeine. No long-term effects of caffeine on blood pressure, heart rate, plasma renin activity, plasma catecholamines, or urinary catecholamines could be demonstrated. Discontinuation of caffeine ingestion after 7 d of administration did not result in a detectable withdrawal phenomenon relating to any of the variables assessed."