I think if you took the Rook (6. Qxa7) Black would respond 6. ... Qd1+ and force a perpetual check draw.  No "mate later."

OMG, figured out the entire exercise without using the solution button. I'm so proud 

SUPER DUPER LONG! :/

easy

Visualization of a seven move mate is at the most highest level of chess play.   Don't you all agree?

I think YaZoOFF has something there with Qf4+ as white's 3rd move.

It is not mate in 5 if black responds 3. ... Ke7 instead of 3. ... Kf7.

But let's see where he can go from there. (*=forced move)
3. ... Ke7, 4. Bg5+ Ke6*, 5 Bd5+  Kd7*, 6. Qf7+ Kd6*,  7. Bf4#

All Black's moves after the third were forced.  Black's alternatives on the 3rd move were Kg6 (and YaZoOff has shown how that ends) ... and ...
3. ... Kg8, 4. Qf8#                                                       ... and ...
3  ... Ke6, 4. Bd5+ Ke7  5. Qf7+  Kd8  6. Bg5+  Re7,  7. Qxe7#   .... or  ....
3  ... Ke6, 4. Bd5+ Ke7  5. Qf7+  Kd6  6. Bf4#                       

I may have missed a variation or two, but I don't think black can avoid mate in 7 or less if white plays 3. Qf4+ instead of the bishop sacrifice at e8.

nice

Took just 1.5 seconds to solve this without mistakes. So eeasyyy.

Everything seems to have been covered except the one defense not mentioned on 3. Qf4+, which is 3...Bf5.  I don't know for sure if black can hold on longer than seven moves, and it doesnt' look very promising, but with the extra possible escape square on c8, now vacated by the bishop, perhaps black can delay getting checkmated within seven moves.  Perhaps someone can find the quickest checkmate possible on 3...Bf5, if there is a mate at all, as I tend to have trouble with mating patterns with the king not confined to an edge of the board. 

good puzzle

No.

not knowing how many moves would be better, just knowing there is a forced mate would be enough.

good puzzle

Loved it!

I'm sure GMs do all the time. It's not so hard if the moves are forced, but I agree such a long forced line doesn't come up very often.

Actually, that is quite slow by modern standards. I managed to solve it in 371 milliseconds.

I wanted to ask what if 5. ... Kd6, not 6. ... Kd6, and then i think that white can in best case get a perpetual check draw, if that 

a difficult one, but very instructive

good for more move puzzle