How often are subsequent investments in most companies stock required? i.e., how often must I reinvest?

For example, many companies have a minimum intial investment of $1000 or $2500, and require minimum subsequent investments of $50 or more. Is there much variation in when the subsequent investment must be made? (sorry, I'm a novice at this). Jimbo

I think there might be a misunderstanding. Open end mutual funds indeed due have a minimum investment amount. Stocks however in the markets I trade do not. You can be as little as one share. Sharebuilder.com even allows buying fractional shares so I understand.

Getting back to open end mutual funds, when they say that subsequent envestment amounts are $50 or whatever they mean that if you wish to invest more money at some time in the future, you must invest at least that amount. You do not have to invest any more at all if you do not wish to. Same goes for stocks. You can invest just one time if you so desire. Now mutual funds do allow you to reinvest the dividend. Also many stocks allow that too. And the reinvestment amount does not have to be the minimum. It is whatever the dividend is.

I'm pretty sure you can just buy one share of stock in a company

Its wise to do "dollar cost averaging"

you don't have to buy any more once you buy some though

No, you can reinvest whenever you want to. Many firms set a minimum reinvestment amount because they don't want to bother with small amounts of dollars. But you make the reinvestment schedule, they don't.