Usage rules for spelling out numbers varies, but most people write out numbers which can be written in one or two words.  Figures are used for numbers which require more words.  See the following examples:

She gained almost ten pounds.

He owed her thirty-five dollars.

There were around seventy people at the party.

Here are some situations where using figures instead of spelling out numbers would be more appropriate:

She weighed almost 245 pounds.

We purchased the guitar for only $750.99.

The facility held almost 1,200 people.

The same rule applies when expressing dates:

April 2, 1987 was her Birthday.

The year was 1969.

That happened in the eighties, I believe.

Here are some different ways in which time can be expressed:

She arrived at 8:00 a.m.

She arrived at eight o'clock in the morning.

Addresses should be written in the following form:

689 South 32 East

42 Eighth Street.

Decimals and Percentages may be written as follows:

She has an 86.3 percent.

It weighs .264.

The recipe called for 3 1/4 cups of sugar.

Large rounded numbers may be written as follows:

She won two million dollars.

She won $2 million.