This can be achieved, without altering the text in the cell, using a Macro. I have to admit, the implementation will be a little retarded, since you need to do a few things.
1 - Tokenize the entire cell content, and store the tokens into a string array.
There is a problem already. The text located inside the cell is grossly inconsistent. You have -250 ms, - 250 ms, - 1sec, - 1 sec. Due to this, you are going to need to check for each instance. Therefore, you cannot just check if the value is a number, you need to check all the permutations that your team members have conveniently provided for you.
The best way to do this would be to check against Regular Expressions. Therefore, #2 should be:
2 - Check each value to see if it matches to one of many 4 Regular Expressions.
3 - Depending on the Regex it matches with, extract the numerical value.
a) If simply a value (ie->250), check if ms or sec proceeds the value. IF ms, divide the number by 1000. If sec, leave number as is. Add number to total.
b) If a number preceded by a dash(-), remove dash, and repeat step a)
c) If a number proceeded by letters, remove letters and repeat step a)
d) If a number proceeded by a period, do nothing. This is a bullet point.