alltid means 'always'

Difficult to decide what tense the sentence should be in as I don't have the context.

Hope this helps anyway.

The above is a direct translation of 'Jeg har savnet deg, alltid.' Depending on the context, for example where in the letter this sentence appears, it could probably be expressed in a more appropriate manner. The tense would perhaps be different, the word 'alltid' may receive a slightly different translation, for example all the time. I could easily imagine that the following would be a more appropriate translation: I miss you, all the time. However, this phrase, on its own, could reasonably be translated as above.