See below. The new recommended daily allowance (rda), as set in 2010, is based on age, as follows: for those 1-70 years of age, 600 iu daily; for those 71 years and older, 800 iu daily; and for pregnant and lactating women, 600 iu daily. The iom further recommended that serum 25(oh)d levels of 20ng/ml (= 50 nmol/l) is adequate, and levels > 50ng/ml (= 125 nmol/l) could have potential adverse effects. Thank.
To maintain or treat. It depends on your vitamin d level and whether you are trying to correct a deficiency or just maintain levels that are already adequate. If the latter, it may be too much, if the former may be ok or may not be enough. If you are trying to fix a deficiency, your level should be monitored.